Glass for use as substrate for information recording medium, substrate for information recording medium, information recording medium, and their production methods

ABSTRACT

According to one aspect of the present invention, provided is glass for use in substrate for information recording medium, which comprises, denoted as molar percentages, a total of 70 to 85 percent of SiO 2  and Al 2 O 3 , where SiO 2  content is equal to or greater than 50 percent and Al 2 O 3  content is equal to or greater than 3 percent; a total of equal to or greater than 10 percent of Li 2 O, Na 2 O and K 2 O; a total of 1 to 6 percent of CaO and MgO, where CaO content is greater than MgO content; a total of greater than 0 percent but equal to or lower than 4 percent of ZrO 2 , HfO 2 , Nb 2 O 5 , Ta 2 O 5 , La 2 O 3  Y 2 O 3  and TiO 2 ; with the molar ratio of the total content of Li 2 O, Na 2 O and K 2 O to the total content of SiO 2 , Al 2 O 3 , ZrO 2 , HfO 2 , Nb 2 O 5 , Ta 2 O 5 , La 2 O 3 , Y 2 O 3  and TiO 2  ((Li 2 O+Na 2 O+K 2 O)/(SiO 2 +Al 2 O 3 +ZrO 2 +HfO 2 +Nb 2 O 5 +Ta 2 O 5 +La 2 O 3 +Y 2 O 3 +TiO 2 )) being equal to or less than 0.28. Further provided are the substrate for information recording medium, information recording medium and their manufacturing methods according to the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2006-159223, filed on Jun. 8, 2006, which is expresslyincorporated herein by reference in its entirety.

This is a divisional of U.S. patent application Ser. No. 12/303,795,filed May 4, 2009. The entire disclosures of the prior applications areconsidered part of the disclosure of the accompanying divisionalapplication, and are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to glass for use in substrate forinformation recording medium such as a magnetic disk, a substrate forinformation recording medium comprised of the aforementioned glass andan information recording medium provided with the aforementionedsubstrate, as well as their manufacturing methods.

BACKGROUND TECHNIQUE

With the development of electronic technology, particularlyinformation-related technology typified by computers, the demand forinformation recording media such as magnetic disks, optical disks, andmagneto-optical disks has increased greatly. The principal structuralcomponents of the magnetic recording devices of computers and the likeare magnetic recording media and magnetic heads for magnetic recordingand reproduction. Magnetic recording media in the forms of flexibledisks and hard disks are known. Hard disk (magnetic disk) substrates maybe made of a variety of materials; there are aluminum, glass, ceramic,and carbon substrates. In practical terms, aluminum substrates and glasssubstrates are primarily employed depending on size and the applicationinvolved. However, as the hard disk drives of notebook computers havedecreased in size and the density of magnetic recording has increased,the requirements of surface smoothness and thickness reduction in disksubstrates have become ever more stringent. Therefore, the limitationsof aluminum substrates, with their poor processability, strength, andrigidity, have been revealed. Accordingly, glass substrates for magneticdisks having good strength, rigidity, impact resistance, and surfacesmoothness have appeared in recent years (for example, see JapaneseExamined Patent Publication (KOKOKU) Showa No. 47-1949 and JapaneseUnexamined Patent Publication (KOKAI) Heisei Nos. 5-32431 and 10-1329,which are expressly incorporated herein by reference in their entirety).

DISCLOSURE OF THE INVENTION

Perpendicular magnetic recording methods have been adopted in recentyears to achieve even greater recording density in information recordingmedia (such as high recording densities of equal to or greater than 100Gbits/inch²). The adoption of perpendicular magnetic recording methodscan substantially increase recording density. Additionally, achievinghigher recording densities requires greatly reducing the distance (orflying height in the case of a magnetic recording medium) between thedata reading and writing head (a magnetic head, for example) and themedium surface to equal to or less than 8 nm. However, a low degree ofsmoothness on the substrate surface causes irregularities on thesubstrate surface to be reflected onto the medium surface, precluding areduction in the distance between the head and the medium surface andpreventing improvement in linear recording density. Thus, achieving highrecording densities through the adoption of perpendicular magneticrecording methods requires a glass substrate for information recordingmedia affording substantially greater smoothness than in the past. As aspecific example, there is an extremely stringent requirement that theroughness of the main surface of the substrate be equal to or less than0.25 nm.

Since the adhesion of foreign matter to glass substrates for informationrecording media is not permitted, adequate cleaning is conducted. Acleaning agent such as an acid or alkali is employed in cleaning.However, when the chemical durability (resistance to acidity,alkalinity, or water) of the glass composing the substrate isinadequate, the substrate surface ends up being roughened during themanufacturing process even when it has been finished to an adequatedegree of smoothness. Even slight surface roughness makes it difficultto achieve the level of smoothness required by a substrate for a mediumin perpendicular recording methods. Thus, enhancing the linear recordingdensity of an information recording medium requires that the substratematerial have good chemical durability.

In order to achieve high-density recording, it is desirable to enhancethe track density in addition to increasing the linear recordingdensity. In disk-shaped information recording media, data are read andrecorded in the direction of rotation while slightly varying thedistance from the center axis, with the area around the center axis ofthe medium is being rotated at high speed. The above linear recordingdensity is an indicator of how many bits of data can be recorded perunit length in the direction of rotation. By contrast, the track densitycorresponds to a recording density in the direction of moving radius ofthe medium. In disk-shaped information recording media, positions onwhich data are recorded are allocated in advance depending on thedistance from the center axis. However, in media with high trackdensities, even a slight displacement of the distance will cause errors.Thus, the center hole in which the rotation axis is installed to rotatethe medium must be precisely formed in the center of the substrate, andthe dimensional tolerance of the inner diameter of the hole must bequite low. In addition to the above, the reason why extremely strictprecision management of the dimensional error of the inner diameter isrequired in a magnetic disk is that the dimensional error of the innerperimeter edge surface of the magnetic disk directly affects theprecision setting when placing the magnetic disk on an HDD spindlemotor. When there is a large inner diameter dimensional error, there isa possibility of inducing mechanical error in the stacking servo(writing of servo information onto the magnetic disk) implemented beforeplacing the magnetic disk onto a magnetic disk device such as an HDD,and thus the possibility of inducing alignment problems with the spindleduring disk stacking. When such problems occur, it becomes impossible torecord or reproduce data. In particular, as information has beenrecorded at ever greater densities in recent years, the distance betweenthe tracks on magnetic disks has decreased. Specifically, as thedistance between tracks (write tracks) has narrowed to equal to or lessthan 0.2 micrometer, only a slight displacement of the substrate causesa shift in the information recording tracks, precluding the correctreading of information.

The current inner diameter tolerance specification is 20.025 mm±0.025 mmin a φ65 mm (65 nm diameter) substrate and 12.025 mm±0.025 mm in a φ48mm substrate. However, as the recording density increases in the future,even more stringent specifications are anticipated.

In glass substrates for information recording media, chemicalstrengthening is sometimes conducted to prevent damage during theprocess of manufacturing the information recording medium and assemblyof the medium into an information recording device. In chemicalstrengthening, in general, glass that contains an alkali component isimmersed in molten salt containing an alkali with a greater ion radiusthan the above alkali component, and the alkali ions on the surface ofthe substrate are exchanged for the alkali ions in the molten salt toform a compression stress layer on the substrate surface. In the courseof chemical strengthening, a large number of substrates are immersed inthe molten salt one after another to conduct the ion exchange. As thenumber of substrates to be processed increases, the concentration of thealkali ions released into the molten salt by the glass increases. Thus,there is a slight difference in stress distribution created by chemicalstrengthening between substrates that are initially chemicallystrengthened and substrates that are chemically strengthened usingmolten salt that has been employed for the treatment of a large numberof other substrates, even when the treatment conditions themselves arekept constant. The glass substrate varies slightly in size before andafter strengthening due to internal stress generated by the chemicalstrengthening. Thus, in substrates that have slightly varying stressdistributions, there is also variation in the change in size. When thisvariation in change in size occurs, the position of the center hole mayshift in individual substrates, albeit slightly, and the tolerance inthe size of the inner diameter of the center hole may increase. Ininformation recording media for high density recording, even such slightshifts can sometimes cause problems, such as precluding the recording ofdata. When the dimensional tolerance in the inner diameter of the centerhole of an information recording medium is large, the medium is moved byimpact, causing the center to shift during operation after having beensecured with a clamping mechanism on the center hole. Even when thiscenter shift is minute, it causes major problems in high recordingdensity information substrates such as perpendicular recording-modemagnetic recording media.

Further, alkali metals are incorporated into the glass to impart ionexchangeability to glass substrates and to enhance the meltingproperties of the glass. However, depending on the content of thesealkali metals, alkali metal ions sometimes leach out of the glasssubstrate. When alkali metal ions leach out and migrate to the surfaceof the glass substrate, they cause problems by moving to the surface andleaching out during the heating step when forming a magnetic film,corroding the magnetic film, or compromising the strength of adhesion ofthe magnetic film.

Under such circumstances, the first object of the present invention isto provide glass for use in substrate for information recording mediumhaving good resistance to acidity and alkalinity, and to provide a glasssubstrate for information recording medium comprised of this glass.

The second object of the present invention is to provide glass for usein substrate for information recording medium from which few alkalimetal components leach out and to which good impact resistance can beimparted by chemical strengthening, and a glass substrate forinformation recording medium comprised of this glass.

The third object of the present invention is to provide glass permittingto produce a substrate for information recording medium that isextremely smooth and of which surface is extremely clean, and a glasssubstrate for information recording medium comprised of this glass.

The fourth object of the present invention is to provide glasspermitting the manufacturing of a substrate for information recordingmedium having high chemical durability and good surface smoothness aftercleaning.

A further object of the present invention is to provide a glasssubstrate material of high shape stability following chemicalstrengthening treatment.

A still further object of the present invention is to provide a methodof manufacturing the various above-described glass substrates forinformation recording medium, and an information recording mediumprovided with these glass substrates and a method of manufacturing thesame.

The present invention relates to glass (referred to as “Glass I”,hereinafter) for use in substrate for information recording medium,which comprises, denoted as molar percentages,

a total of 70 to 85 percent of SiO₂ and Al₂O₃, where SiO₂ content isequal to or greater than 50 percent and Al₂O₃ content is equal to orgreater than 3 percent;

a total of equal to or greater than 10 percent of Li₂O, Na₂O and K₂O;

a total of 1 to 6 percent of CaO and MgO, where CaO content is greaterthan MgO content;

a total of greater than 0 percent but equal to or lower than 4 percentof ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ Y₂O₃ and TiO₂;

with the molar ratio of the total content of Li₂O, Na₂O and K₂O to thetotal content of SiO₂, Al₂O₃, ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂))being equal to or less than 0.28.

According to one embodiment, in the aforementioned glass, SiO₂ contentis equal to or greater than 60 molar percent, and the total content ofSiO₂ and Al₂O₃ is equal to or greater than 75 molar percent.

According to one embodiment, in the aforementioned glass, denoted asmolar percentages, the total content of CaO and MgO is greater than thetotal content of SrO and BaO, and the molar ratio of the total contentof ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ to the total contentof Li₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO)) isequal to or greater than 0.035.

According to one embodiment, the aforementioned glass comprises, denotedas molar percentages, 50 to 75 percent of SiO₂, 3 to 15 percent ofAl₂O₃, 5 to 15 percent of Li₂O, 5 to 15 percent of Na₂O, 0 to 3 percentof K₂O, greater than 0.5 percent but equal to or less than 5 percent ofCaO, equal to or greater than 0 percent but less than 3 percent of MgO,and 0.3 to 4 percent of ZrO₂.

Another aspect of the present invention relates to glass (referred to as“Glass II”, hereinafter) for use in substrate for information recordingmedium, which comprises, denoted as molar percentages,

50 to 75 percent of SiO₂;

3 to 15 percent of Al₂O₃;

5 to 15 percent of Li₂O;

5 to 15 percent of Na₂O;

0 to 3 percent of K₂O;

greater than 0.5 percent but equal to or less than 5 percent of CaO;

equal to or greater than 0 percent but less than 3 percent of MgO, withCaO content being greater than MgO content; and

0.3 to 4 percent of ZrO₂;

with the molar ratio of the total content of Li₂O, Na₂O and K₂O to thetotal content of SiO₂, Al₂O₃ and ZrO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂)) being equal to or less than 0.28.

A further aspect of the present invention relates to glass (referred toas “Glass III”, hereinafter) for use in substrate for informationrecording medium, which comprises SiO₂; Al₂O₃; one or more alkali metaloxides selected from the group consisting of Li₂O, Na₂O and K₂O; one ormore alkaline earth metal oxides selected from the group consisting ofMgO, CaO, SrO and BaO; and one or more oxides selected from the groupconsisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

wherein SiO₂ content is equal to or greater than 50 molar percent, andthe total content of SiO₂ and Al₂O₃ is equal to or greater than 70 molarpercent;

the total content of the above alkali metal oxides and the abovealkaline earth meal oxides is equal to or greater than 8 molar percent;and

the molar ratio of the total content of the above oxides to the totalcontent of the above alkali metal oxides and the above alkaline earthmetal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO)) isequal to or greater than 0.035.

According to one embodiment, in the aforementioned glass, SiO₂ contentis equal to or greater than 60 molar percent, and the total content ofSiO₂ and Al₂O₃ is equal to or greater than 75 molar percent.

According to one embodiment, the aforementioned glass comprises at leastone of Li₂O and Na₂O, and the total content of Li₂O and Na₂O is lessthan 24 molar percent.

According to one embodiment, in the aforementioned glass, the totalcontent of Li₂O and Na₂O is equal to or less than 22 molar percent.

According to one embodiment, the aforementioned glass comprises, denotedas molar percentages, 60 to 75 percent of SiO₂, 3 to 15 percent ofAl₂O₃, and 0.3 to 4 percent of ZrO₂.

A further aspect of the present invention relates to aluminosilicateglass (referred to as “Glass IV”, hereinafter) for chemicalstrengthening for use in substrate for information recording medium,which comprises:

one or more alkali metal oxides selected from the group consisting ofLi₂O, Na₂O and K₂O, one or more alkaline earth metal oxides selectedfrom the group consisting of MgO, CaO, SrO and BaO, and one or moreoxides selected from the group consisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅,La₂O₃, Y₂O₃ and TiO₂;

wherein the total content of Li₂O and Na₂O is 10 to 22 molar percent;

the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ isgreater than 0 molar percent but equal to or less than 4 molar percent;and

the molar ratio of the total content of the above oxides to the totalcontent of the above alkaline earth metal oxides((ZrO₂+HCO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(MgO+CaO+SrO+BaO)) is equal toor greater than 0.15.

According to one embodiment, in the aforementioned glass, SiO₂ contentis equal to or greater than 50 molar percent, and the total content ofSiO₂ and Al₂O₃ is equal to or greater than 70 molar percent.

According to one embodiment, in the aforementioned glass, the totalcontent of SiO₂ and Al₂O₃ is equal to or greater than 75 molar percent.

According to one embodiment, in the aforementioned glass, the molarratio of the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂ to the total content of Li₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035.

According to one embodiment, the aforementioned glass comprises, denotedas molar percentages, equal to or greater than 3 percent of Al₂O₃, atotal of equal to or greater than 8 percent of Li₂O, Na₂O, K₂O, MgO,CaO, SrO and BaO, and a total of greater than 0 percent but equal to orless than 5 percent of MgO, CaO, SrO and BaO.

A further aspect of the present invention relates to glass (referred toas “Glass V”, hereinafter) for use in substrate for informationrecording medium, which comprises SiO₂; Al₂O₃; one or more alkali metaloxides selected from the group consisting of Li₂O, Na₂O and K₂O; one ormore alkaline earth metal oxides selected from the group consisting ofMgO, CaO, SrO and BaO; and one or more oxides selected from the groupconsisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

which has an acidity resistance resulting in an etching rate of equal toor less than 3.0 nm/minute when immersed in 0.5 volume percenthydrogenfluosilicic acid (H₂SiF) aqueous solution maintained at 50° C.;and

which has an alkalinity resistance resulting in an etching rate of equalto or less than 0.1 nm/minute when immersed in 1 mass percent potassiumhydroxide aqueous solution maintained at 50° C.

According to one embodiment, in the aforementioned glass, SiO₂ contentis equal to or greater than 50 molar percent, and the total content ofSiO₂ and Al₂O₃ is equal to or greater than 70 molar percent.

According to one embodiment, in the aforementioned glass, the totalcontent of SiO₂ and Al₂O₃ is equal to or greater than 75 molar percent.

According to one embodiment, the aforementioned glass has a compositionthat the total content of the above alkali metal oxides and the abovealkaline earth metal oxides is equal to or greater than 8 molar percent,and the molar ratio of the total content of the above oxides to thetotal content of the above alkali metal oxides and the above alkalineearth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035.

According to one embodiment, the aforementioned glass comprises at leastone of Li₂O and Na₂O, and the total content of Li₂O and Na₂O is equal toor less than 24 molar percent.

According to one embodiment, in the aforementioned glass, the totalcontent of Li₂O and Na₂O is equal to or less than 22 molar percent.

According to one embodiment, the aforementioned glass comprises, denotedas molar percentages, 60 to 75 percent of SiO₂, 3 to 15 percent ofAl₂O₃, and a total of 0.3 to 4 percent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅,La₂O₃, Y₂O₃ and TiO₂.

A further aspect of the present invention relates to glass (referred toas “Glass VI”, hereinafter) for use in substrate for informationrecording medium, which comprises, denoted as mass percentages,

57 to 75 percent of SiO₂;

5 to 20 percent of Al₂O₃, with the total content of SiO₂ and Al₂O₃ beingequal to or greater than 74 percent;

a total of greater than 0 percent but equal to or less than 6 percent ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

greater than 1 percent but equal to or less than 9 percent of Li₂O;

5 to 18 percent of Na₂O, with the mass ratio, Li₂O/Na₂O being equal toor less than 0.5;

0 to 6 percent of K₂O;

0 to 4 percent of MgO;

greater than 0 percent but equal to or less than 5 percent of CaO, withthe total content of MgO and CaO being equal to or less than 5 percentand CaO content being greater than MgO content; and

a total of 0 to 3 percent of SrO and BaO; as well as

glass (referred to as “Glass VII”, hereinafter) for use in substrate forinformation recording medium, which comprises, denoted as masspercentages,

57 to 75 percent of SiO₂;

5 to 20 percent of Al₂O₃, with the total content of SiO₂ and Al₂O₃ beingequal to or greater than 74 percent;

greater than 0 percent but equal to or less than 5.5 percent of ZrO₂;

greater than 1 percent but equal to or less than 9 percent of Li₂O;

5 to 18 percent of Na₂O, with the mass ratio, Li₂O/Na₂O being equal toor less than 0.5;

0 to 6 percent of K₂O;

0 to 4 percent of MgO;

greater than 0 percent but equal to or less than 5 percent of CaO, withthe total content of MgO and CaO being equal to or less than 5 percentand CaO content being greater than MgO content;

a total of 0 to 3 percent of SrO and BaO; and

0 to 1 percent of TiO₂.

According to one embodiment, in the aforementioned glass, the totalcontent of SiO₂ and Al₂O₃ is greater than 79 percent.

According to one embodiment, the aforementioned glass comprises equal toor greater than 11 percent of Al₂O₃.

According to one embodiment, the aforementioned glass comprises 0.1 to 4percent of MgO.

According to one embodiment, in the aforementioned glass, the totalcontent of SiO₂, Al₂O₃, ZrO₂, Li₂O, Na₂O, K₂O, MgO and CaO is equal toor greater than 99 percent.

According to one embodiment, each of the aforementioned glasses canfurther comprise Fe.

A further aspect of the present invention relates to chemicallystrengthened glass for use in substrate for information recordingmedium, obtained by subjecting the aforementioned glass to chemicalstrengthening treatment.

A further aspect of the present invention relates to a glass substratefor information recording medium being comprised of the aforementionedglass.

According to one embodiment, the aforementioned glass substrate has amain surface with a roughness Ra of less than 0.25 nm.

According to one embodiment, the aforementioned glass substrate has beensubjected to chemical strengthening treatment.

According to one embodiment, the aforementioned glass substrate has adeflecting strength of equal to or greater than 10 kg.

According to one embodiment, the aforementioned glass substrate has athickness of equal to or less than 1 mm.

According to one embodiment, the aforementioned glass substrate has athickness of equal to or greater than 0.3 mm.

According to one embodiment, the aforementioned glass substrate isdisk-shaped, and has a hole in the center.

A further aspect of the present invention relates to a method ofmanufacturing a glass substrate for information recording medium, whichcomprises the steps of mirror finishing the aforementioned glass, andfollowing mirror polishing, conducting acid cleaning and alkalicleaning.

According to one embodiment, the aforementioned manufacturing methodfurther comprises chemical strengthening treatment between the abovemirror finishing step and cleaning step.

According to one embodiment, the acid cleaning and the alkali cleaningare sequentially conducted.

According to one embodiment, the alkali cleaning is conducted followingthe acid cleaning.

A further aspect of the present invention relates to an informationrecording medium comprising an information recording layer on theaforementioned glass substrate for information recording medium.

According to one embodiment, the aforementioned information recordingmedium is a perpendicular magnetic recording-mode magnetic recordingmedium.

According to one embodiment, the aforementioned information recordingmedium which has a soft magnetic underlayer, an amorphous underlayer, acrystalline underlayer, a perpendicular magnetic recording layer, aprotective layer and a lubricating layer in this order on the abovesubstrate.

According to one embodiment, the aforementioned information recordingmedium has a recording density of equal to or greater than 130Gbit/inch².

A further aspect of the present invention relates to a method ofmanufacturing an information recording medium, wherein a glass substratefor information recording medium is manufactured by the aforementionedmethod, and an information recording layer is formed on the glasssubstrate.

The present invention can provide glass for use in substrate forinformation recording medium affording good resistance to acidity andalkalinity; a glass substrate for information recording medium comprisedof this glass; a method of manufacturing the same; an informationrecording medium provided with this substrate; and a method ofmanufacturing the same.

The present invention further can provide glass for use in substrate forinformation recording medium, from which few alkali metal componentsleach out and to which good impact resistance can be imparted bychemical strengthening; a glass substrate for information recordingmedium comprised of this glass; a method of manufacturing the same; aninformation recording medium provided with this substrate; and a methodof manufacturing the same.

The present invention can further provide glass permitting to produce asubstrate for information recording medium that is extremely smooth andof which surface is extremely clean, and a glass substrate forinformation recording medium comprised of this glass; a method ofmanufacturing the same; an information recording medium provided withthis substrate; and a method of manufacturing the same.

Furthermore, the present invention can provide a substrate forinformation recording medium made of glass having excellent surfacesmoothness that is suited to high recording densities with a distancebetween tracks of equal to or less than 0.2 micrometer, desirably equalto or less than 0.15 micrometer, and preferably equal to or less than0.12 micrometer, for example; a method of manufacturing the same; aninformation recording medium provided with this substrate; and a methodof manufacturing the same.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in greater detail below.

The present invention relates to glasses for use in substrate forinformation recording medium. The glasses for use in substrate forinformation recording medium of the present invention (also referred toas “glasses for information recording medium substrate”, hereinafter)can be broadly divided into the seven glasses of above-mentioned glassesI to VII. These glasses will be sequentially described in detail below.

[Glass I]

Glass I is glass for use in substrate for information recording medium,which comprises, denoted as molar percentages,

a total of 70 to 85 percent of SiO₂ and Al₂O₃, where SiO₂ content isequal to or greater than 50 percent and Al₂O₃ content is equal to orgreater than 3 percent;

a total of equal to or greater than 10 percent of Li₂O, Na₂O and K₂O;

a total of 1 to 6 percent of CaO and MgO, where CaO content is greaterthan MgO content;

a total of greater than 0 percent but equal to or lower than 4 percentof ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ Y₂O₃ and TiO₂;

with the molar ratio of the total content of Li₂O, Na₂O and K₂O to thetotal content of SiO₂, Al₂O₃, ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂))being equal to or less than 0.28.

Glass I can provide a substrate for information recording medium havinggood resistance to both acidity and alkalinity.

Unless specifically noted otherwise, the individual component contentsand total contents given below in the description of Glass I are denotedas molar percentages, and ratios between contents are given as molarratios. Glass I is an oxide glass and the contents of the individualcomponents are denoted as their values when converted to oxides.

SiO₂, a glass network-forming component, is an essential componentfunctioning to increase the stability, chemical durability, and inparticular, resistance to acidity of the glass; to lower the thermaldiffusion of the substrate; and to raise the heating efficiency of thesubstrate by radiation.

Al₂O₃ also contributes to the formation of a glass network, andfunctions to increase glass stability and chemical durability.

In Glass I, a total content of SiO₂ and Al₂O₃ is equal to or greaterthan 70 percent, preferably equal to or greater than 74 percent, andmore preferably, equal to or greater than 75 percent to increasechemical durability, and in particular, resistance to acidity. Inconsideration of the melting properties of the glass, a total content ofSiO₂ and Al₂O₃ is equal to or less than 85 percent, preferably equal toor less than 80 percent.

To achieve good glass stability, SiO₂ content is equal to or greaterthan 50 percent, preferably equal to or greater than 55 percent, morepreferably equal to or greater than 60 percent, further preferably equalto or greater than 63 percent, and still more preferably, equal to orgreater than 65 percent. However, the incorporation of an excessivequantity of SiO₂ produces unmelted material in the glass. Thus, thequantity of SiO₂ is preferably kept to equal to or less than 75 percent,more preferably equal to or less than 72, and further preferably, equalto or less than 70 percent. When glass in which unmelted material ispresent is processed into a substrate, portions of the unmelted materialare sometimes exposed on the surface of the substrate, formingprotrusions. Substrates having such protrusions cannot be employed assubstrates for information recording media in which a high degree ofsmoothness is required. Accordingly, the melting property of a glassemployed in a substrate for information recording media is an importantcharacteristic.

Al₂O₃ content is equal to or greater than 3 percent, desirably equal toor greater than 5 percent, and preferably, equal to or greater than 7percent. However, the introduction of an excess of Al₂O₃ compromises themelting property of the glass. Thus, the Al₂O₃ content is desirablyequal to or less than 15 percent, preferably equal to or less than 12percent.

Li₂O, Na₂O, and K₂O are components that are useful for enhancing meltingproperties and formability, as well as increasing the coefficient ofthermal expansion to impart suitable thermal expansion characteristicsto substrates for information recording media, particularly substratesfor magnetic recording media. When Li₂O and Na₂O are employed inchemically strengthened glasses, they function as ion exchangecomponents during chemical strengthening. To achieve such effects, thetotal content of Li₂O, Na₂O and K₂O is set to equal to or greater than10 percent. However, when the quantity of alkali metal oxides isexcessive, chemical durability, and, particularly, resistance toacidity, tend to decrease. Accordingly, in Glass I, from the perspectiveof enhancing chemical durability, the upper limit to the total contentof Li₂O, Na₂O and K₂O is determined in relation to the total content ofSiO₂, Al₂O₃, ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂. Thisrelation will be described in detail further below. To further enhancechemical durability, the total content of Li₂O, Na₂O and K₂O isdesirably kept to equal to or less than 22 percent, preferably equal toor less than 21.5 percent, more preferably equal to or less than 21percent, and still more preferably, equal to or less than 20 percent.

To reduce and prevent the leaching out of alkali metal componentsthrough an effect achieved by mixing alkalis, Li₂O and Na₂O aredesirably incorporated as glass components.

To enhance the various above-described characteristics, the lower limitof the Li₂O content is desirably 5 percent, preferably 6 percent, andmore preferably, 7 percent; and the upper limit is desirably 15 percent,preferably 13 percent, and more preferably, 10 percent. The lower limitof the Na₂O content is desirably 5 percent, preferably 7 percent, andmore preferably, 10 percent; and the upper limit is desirably 15percent, preferably 13 percent.

When employing glass containing Li₂O and Na₂O as chemically strengthenedglass, Li₂O and Na₂O are glass components that directly contribute toion exchange during the chemical strengthening treatment. In moltensalt, the alkali ions contributing to ion exchange are Na ions and/or Kions. As the number of substrates that have been subjected to chemicalstrengthening treatment increases, the concentration of Li ions in themolten salt increases. However, during the treatment of a large quantityof a glass in which the molar ratio of the quantity of Li₂O to that ofNa₂O (Li₂O/Na₂O) exceeds 1.04, the increase in the concentration of Liions in the molten salt becomes pronounced, and the balance between thealkali ions contributing to ion exchange and the alkali ions notcontributing to ion exchange changes greatly relative to what it is atthe start of processing. As a result, as the number of substrates thathave been treated increases, treatment conditions that were optimal atthe start of treatment move outside the optimal range. As set forthabove, there are sometimes problems in the form of variation in theshape of the substrates, increased dimensional tolerance in the innerdiameter of the substrate center hole, inadequate formation of thecompression stress layer, the development of waviness in the substrate,or the like. To solve such problems, the molar ratio of the quantity ofLi₂O to that of Na₂O (Li₂O/Na₂O) is desirably kept to equal to or lessthan 1.04, preferably equal to or less than 0.936, more preferably equalto or less than 0.832, and still more preferably, equal to or less than0.7904.

K₂O is an optional component functioning to enhance melting propertiesand raise the coefficient of thermal expansion. The desirable range ofK₂O content is 0 to 3 percent, preferably 0 to 2 percent, and morepreferably, 0 to 1 percent. When K₂O is incorporated in a smallquantity, it has the effect of reducing variation in the compressionstress layer between substrates during the chemical strengthening oflarge numbers of substrates. Thus, within the above-stated range, it isdesirably introduced in a quantity of equal to or greater than 0.1percent, preferably equal to or greater than 0.2 percent.

CaO and MgO improve melting properties, formability, and glassstability; enhance rigidity and hardness; and raise the coefficient ofthermal expansion. However, chemical durability decreases when anexcessive quantity is introduced. Thus, the total content of CaO and MgOis kept to 1 to 6 percent. The lower limit of the total content of CaOand MgO is desirably 1.5 percent, preferably 2 percent; the upper limitis desirably 5.5 percent, preferably 5 percent, and more preferably, 4percent. CaO and MgO function to reduce the rate of ion exchange duringchemical strengthening. Accordingly, an increase in the inner and outerdimensional tolerances of the substrate caused by excessive chemicalstrengthening can be prevented during the mass production of chemicallystrengthened glass substrates by chemically strengthening substratescomprised of glass in which suitable quantities of these components havebeen incorporated. However, when excessive quantities of thesecomponents are incorporated, the ion exchange rate decreases greatly,making it difficult to achieve a chemical strengthening effect.Incorporating CaO and MgO within the above-stated total content rangeprevents an increase in the inner and outer diameter dimensiontolerances of the substrate while achieving a chemical strengtheningeffect.

In Glass I, CaO content is set to greater than MgO content to furtherincrease resistance to devitrification and increase chemical durability.To further increase resistance to devitrification and increase chemicaldurability, the molar ratio of MgO content to CaO content (MgO/CaO)desirably falls within a range of 0.14 to 0.97, preferably a range of0.4 to 0.97.

The CaO content is desirably greater than 0.5 percent but equal to orless than 5 percent. The lower limit of the CaO content is desirably 0.8percent, preferably 1 percent; the upper limit is desirably 4 percent,preferably 3 percent.

The MgO content is desirably equal to or greater than 0 percent, butless than 3 percent. The lower limit of the MgO content is 0.1 percent,preferably 0.3 percent, and more preferably, 0.5 percent. The upperlimit is desirably 2.5 percent, preferably 2 percent. The MgO contentmay be determined based on the CaO content that has been firstdetermined and the molar ratio (MgO/CaO) that has been then set withinthe above-stated preferable range.

SrO and BaO, which are both alkaline earth metal oxides just like CaOand MgO, function to enhance melting properties and raise thecoefficient of thermal expansion. However, the addition of SrO and BaOdecreases chemical durability, increases the specific gravity of theglass, and tends to increase the cost of the starting materials. Thus,the total content of CaO and MgO is desirably greater than the totalcontent of SrO and BaO.

The total content of SrO and BaO is desirably 0 to 5 percent, preferably0 to 2 percent, and more preferably, 0 to 1 percent. The SrO contentdesirably falls within a range of 0 to 2 percent, preferably 0 to 1percent, with no incorporation of SrO being of even greater preference.The BaO content desirably falls within a range of 0 to 2 percent,preferably 0 to 1 percent, with no incorporation of BaO being of evengreater preference.

ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ function to increasechemical durability, particularly resistance to alkalinity. However, theincorporation of an excessive quantity compromises melting properties.Accordingly, the total contents of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃and TiO₂ are kept to greater than 0 but equal to or less than 4 percentto increase chemical durability, particularly resistance to alkalinitywhile maintaining melting properties. The lower limit of this totalcontent is desirably 0.3 percent, preferably 0.5 percent, and morepreferably, 0.7 percent; the upper limit is desirably 3 percent,preferably 2 percent, and more preferably, 1.5 percent.

To further increase chemical durability, particularly resistance toalkalinity, the relation between the total content of alkaline earthmetal oxides and alkali metal oxides, which enhance melting propertiesbut tend to lower chemical durability, and the total content of , ZrO₂,HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂, desirably falls within thefollowing range.

That is, the molar ratio of the total content of, ZrO₂, HfO₂, Nb₂O₅,Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ to the total content of Li₂O, Na₂O, K₂O,MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is desirably equal to or greater than 0.035, preferably equal to orgreater than 0.040. However, when this molar ratio becomes excessivelylarge, the melting properties tend to deteriorate and/or the glass tendsto destabilize. Thus, this molar ratio is desirably equal to or lessthan 0.18, preferably equal to or less than 0.15, more preferably equalto or less than 0.13, and still more preferably, equal to or less than0.12.

Among ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂, when glasscontaining TiO₂ is dipped in water, a reaction product of the glass andwater sometimes adheres to the glass surface. Thus, the other componentsare advantageous with regard to resistance to water. Accordingly, tomaintain water resistance, the content of TiO₂ is desirably 0 to 2percent, preferably 0 to 1 percent, more preferably 0 to 0.5 percent,with no incorporation of TiO₂ being of even greater preference.

HfO₂, Nb₂O₅, Ta₂O₅ and La₂O₃ increase the specific gravity of the glassand increase the weight of the substrate. Thus, to lighten thesubstrate, the total content of HfO₂, Nb₂O₅, Ta₂O₅ and La₂O₃ desirablyfalls within a range of 0 to 2 percent, preferably 0 to 1 percent, withno incorporation of HfO₂, Nb₂O₅, Ta₂O₅ or La₂O₃ being of even greaterpreference. The respective contents of HfO₂, Nb₂O₅, Ta₂O₅ and La₂O₃ arepreferably 0 to 2 percent, more preferably 0 to 1 percent, with noincorporation of HfO₂, Nb₂O₅, Ta₂O₅ or La₂O₃ being of even greaterpreference.

In order to produce the above-described desirable effects whilemaintaining glass stability, Y₂O₃ content desirably falls within a rangeof 0 to 2 percent, preferably 0 to 1 percent, with no incorporation ofY₂O₃ being of even greater preference.

ZrO₂ functions to strongly increase chemical durability, particularlyresistance to alkalinity, while maintaining glass stability; enhancesrigidity and toughness; and functions to increase the efficiency ofchemical strengthening. Since it is an inexpensive starting materialrelative to Y₂O₃, the molar ratio of the content of ZrO₂ to the totalcontent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂(ZrO₂/(ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)) desirably falls within arange of 0.5 to 1, preferably 0.8 to 1, more preferably 0.9 to 1, stillmore preferably 0.95 to 1, and is most preferably 1.

ZrO₂ content is desirably equal to or greater than 0.3 percent,preferably equal to or greater than 0.5 percent, and more preferably,equal to or greater than 0.7 percent. To maintain good meltingproperties and glass stability, the content of ZrO₂ is desirably equalto or less than 4 percent, preferably equal to or less than 3 percent,more preferably equal to or less than 2 percent, and still morepreferably, equal to or less than 1.5 percent.

Among the above-described glass components, SiO₂, Al₂O₃, ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ function to enhance chemicaldurability, and Li₂O, Na₂O and K₂O tend to lower chemical durability.Accordingly, in Glass I, the upper limit of the molar ratio of the totalcontent of Li₂O, Na₂O, and K₂O to the total content of SiO₂, Al₂O₃,ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)) islimited to equal to or less than 0.28. This molar ratio is desirablyequal to or less than 0.27, preferably equal to or less than 0.26.

As needed, clarifying agents such as Sb₂O₃, SnO₂ and CeO₂ may be addedto Glass I. When the glass is being formed by the floating method, theaddition of Sb₂O₃ is undesirable, the addition of SnO₂ and CeO₂ isdesirable, and the addition of SnO₂ is preferred.

A preferred embodiment of the above-described Glass I is glasscomprising, denoted as molar percentages, 50 to 75 percent of SiO₂, 3 to15 percent of Al₂O₃, 5 to 15 percent Li₂O, 5 to 15 percent of Na₂O, 0 to3 percent of K₂O, greater than 0.5 percent but equal to or less than 5percent of CaO, equal to or greater than 0 percent but less than 3percent of MgO, and 0.3 to 4 percent of ZrO₂.

[Glass II]

Glass II is glass for use in substrate for information recording medium,which comprises, denoted as molar percentages,

50 to 75 percent of SiO₂;

3 to 15 percent of Al₂O₃;

5 to 15 percent of Li₂O;

5 to 15 percent of Na₂O;

0 to 3 percent of K₂O;

greater than 0.5 percent but equal to or less than 5 percent of CaO;

equal to or greater than 0 percent but less than 3 percent of MgO, withCaO content being greater than MgO content; and

0.3 to 4 percent of ZrO₂;

with the molar ratio of the total content of Li₂O, Na₂O and K₂O to thetotal content of SiO₂, Al₂O₃ and ZrO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂)) being equal to or less than 0.28.

Glass II can provide a substrate for information recording medium withgood resistance to both acidity and alkalinity.

Unless specifically noted otherwise, the individual component contentsand total contents given below in the description of Glass II aredenoted as molar percentages, and ratios between contents are given asmolar ratios. Glass II is an oxide glass and the contents of theindividual components are denoted as their values when converted tooxides.

SiO₂, a glass network-forming component, is an essential componentfunctioning to increase the stability, chemical durability, and inparticular, resistance to acidity of the glass; to lower the thermaldiffusion of the substrate; and to raise the heating efficiency of thesubstrate by radiation. To achieve good glass stability, SiO₂ content isequal to or greater than 50 percent, desirably equal to or greater than55 percent, preferably equal to or greater than 60 percent, morepreferably equal to or greater than 63 percent, and still morepreferably, equal to or greater than 65 percent. However, theincorporation of an excessive quantity of SiO₂ produces unmeltedmaterial in the glass. Thus, the content of SiO₂ is kept to equal to orless than 75 percent, preferably equal to or less than 72 percent, andmore preferably, equal to or less than 70 percent. When glass in whichunmelted material is present is processed into a substrate, portions ofthe unmelted material are sometimes exposed on the surface of thesubstrate, forming protrusions. Substrates having such protrusionscannot be employed as substrates for information recording media inwhich a high degree of smoothness is required. Accordingly, the meltingproperty of a glass employed in a substrate for information recordingmedia is an important characteristic.

Al₂O₃ also contributes to the formation of the glass network, andfunctions to increase glass stability and chemical durability. Toachieve such effects, Al₂O₃ content is equal to or greater than 3percent, preferably equal to or greater than 5 percent, and morepreferably, equal to or greater than 7 percent. However, theincorporation of an excessive quantity of Al₂O₃ compromises the meltingproperties of the glass. Thus, the content of Al₂O₃ is set to equal toor less than 15 percent, desirably equal to or less than 12 percent.

In Glass II, the total content of SiO₂ and Al₂O₃ is desirably equal toor greater than 70 percent, preferably equal to or greater than 74percent, and more preferably, equal to or greater than 75 percent toincrease chemical durability, and in particular, resistance to acidity.In consideration of the melting properties of the glass, the totalcontent of SiO₂ and Al₂O₃ is preferably equal to or less than 85percent, more preferably equal to or less than 80 percent.

Li₂O, Na₂O and K₂O are components that are useful for enhancing meltingproperties and formability, as well as increasing the coefficient ofthermal expansion to impart suitable thermal expansion characteristicsto substrates for information recording media, particularly substratesfor magnetic recording media. When Li₂O and Na₂O are employed inchemically strengthened glasses, they function as ion exchangecomponents during chemical strengthening. To achieve such effects, Li₂Ocontent is 5 to 15 percent, Na₂O content is 5 to 15 percent, and K₂Ocontent is 0 to 3 percent. The lower limit of the Li₂O content isdesirably 6 percent, preferably 7 percent; the upper limit is desirably13 percent, preferably 10 percent. The lower limit of the Na₂O contentis desirably 7 percent, preferably 10 percent; the upper limit isdesirably 13 percent.

By incorporating Li₂O and Na₂O as glass components, the leaching out ofalkali metal components can be reduced or prevented through an effectachieved by mixing alkalis.

When employing glass containing Li₂O and Na₂O as chemically strengthenedglass, Li₂O and Na₂O are glass components that directly contribute toion exchange during chemical strengthening. In molten salt, the alkaliions contributing to ion exchange are Na ions and/or K ions. As thenumber of substrates that have been subjected to chemical strengtheningtreatment increases, the concentration of Li ions in the molten saltincreases. However, as a large quantity of a glass in which the molarratio of the quantity of Li₂O to that of Na₂O (Li₂O/Na₂O) exceeds 1.04is treated, the increase in the concentration of Li ions in the moltensalt becomes pronounced, and the balance between the alkali ionscontributing to ion exchange and the alkali ions not contributing to ionexchange changes greatly relative to what it is at the start ofprocessing. As a result, as the number of substrates that have beentreated increases, treatment conditions that were optimal at the startof treatment move outside the optimal range. As set forth above, thereare sometimes problems in the form of variation in the shape of thesubstrates, increased dimensional tolerance in the inner diameter of thesubstrate center hole, inadequate formation of the compression stresslayer, the development of waviness in the substrate, or the like. Tosolve such problems, the molar ratio of the quantity of Li₂O to that ofNa₂O (Li₂O/Na₂O) is desirably kept to equal to or less than 1.04,preferably equal to or less than 0.936, more preferably equal to or lessthan 0.832, and still more preferably, equal to or less than 0.7904.

K₂O is an optional component functioning to enhance melting propertiesand raise the coefficient of thermal expansion. The range of the K₂Ocontent is 0 to 3 percent, preferably 0 to 2 percent, and morepreferably, 0 to 1 percent. When K₂O is incorporated in a smallquantity, it has the effect of reducing variation in the compressionstress layer between substrates during the chemical strengthening oflarge numbers of substrates. Thus, within the above-stated range, it isdesirably introduced in a quantity of equal to or greater than 0.1percent, preferably equal to or greater than 0.2 percent.

When an excessive quantity of alkali metal oxides is present, there is atendency for chemical durability, and particularly, resistance toacidity to decrease. Accordingly, in Glass II, from the perspective ofenhancing chemical durability, the upper limit of the total content ofLi₂O, Na₂O and K₂O is determined in relation to the total content ofSiO₂, Al₂O₃ and ZrO₂. The details are given further below. To furtherenhance chemical durability, the total content of Li₂O, Na₂O and K₂O isdesirably equal to or less than 22 percent, preferably equal to or lessthan 21.5 percent, more preferably equal to or less than 21 percent, andstill more preferably, equal to or less than 20 percent.

CaO and MgO improve melting properties, formability, and glassstability; enhance rigidity and hardness; and raise the coefficient ofthermal expansion. In particular, CaO functions well to improve meltingproperties, formability, and glass stability. However, the incorporationof an excessive quantity of either of these components reduces chemicaldurability. Thus, the content of CaO is set to greater than 0.5 percentbut equal to or less than 5 percent. The lower limit of the CaO contentis desirably 0.8 percent, preferably 1 percent, and the upper limit isdesirably 4 percent, preferably 3 percent.

MgO content is equal to or greater than 0 percent but less than 3percent. The lower limit of the MgO content is desirably 0.1 percent,preferably 0.3 percent, and more preferably, 0.5 percent; the upperlimit is desirably 2.5 percent, preferably 2 percent.

In Glass II, the content of CaO is set to greater than the content ofMgO to further enhance resistance to devitrification and increasechemical durability. To further enhance resistance to devitrificationand increase chemical durability, the molar ratio of the content of MgOto that of CaO (MgO/CaO) desirably falls within a range of 0.14 to 0.97,preferably a range of 0.4 to 0.97.

The MgO content may be determined based on the CaO content that has beenfirst determined and the molar ratio (MgO/CaO) that has been then setwithin the above-stated preferable range.

The total content of CaO and MgO is desirably from 1 to 6 percent toenhance chemical durability while further improving melting properties,formability, and glass stability. The lower limit of the total contentof CaO and MgO is desirably 1.5 percent, preferably 2 percent; the upperlimit is desirably 5.5 percent, preferably 5 percent, and morepreferably, 4 percent. CaO and MgO function to reduce the rate of ionexchange during chemical strengthening. Accordingly, an increase in theinner and outer dimensional tolerances of the substrate caused byexcessive chemical strengthening can be prevented during the massproduction of chemically strengthened glass substrates by chemicallystrengthening substrates comprised of glass in which suitable quantitiesof these components have been incorporated. However, when excessivequantities of these components are incorporated, the ion exchange ratedecreases greatly, making it difficult to achieve a chemicalstrengthening effect. Incorporating CaO and MgO within the above-statedtotal content range can prevent an increase in the inner and outerdiameter dimension tolerances of the substrate while achieving achemical strengthening effect.

SrO and BaO, which are both alkaline earth metal oxides just like CaOand MgO, function to enhance melting properties and raise thecoefficient of thermal expansion. However, the addition of SrO and BaOdecreases chemical durability, increases the specific gravity of theglass, and tends to increase the cost of the starting materials. Thus,the total content of CaO and MgO is desirably greater than the totalcontent of SrO and BaO.

The total content of SrO and BaO is desirably 0 to 5 percent, preferably0 to 2 percent, and more preferably, 0 to 1 percent. The SrO contentdesirably falls within a range of 0 to 2 percent, preferably 0 to 1percent, with no incorporation of SrO being of even greater preference.The content of BaO desirably falls within a range of 0 to 2 percent,preferably 0 to 1 percent, with no incorporation of BaO being of evengreater preference.

ZrO₂ functions to enhance chemical durability, particularly resistanceto alkalinity, increase rigidity and toughness, and increase theeffectiveness of chemical strengthening while maintaining glassstability. However, the incorporation of an excessive quantitycompromises melting properties. Accordingly, the content of ZrO₂ is setto 0.3 to 4 percent to enhance chemical durability, particularlyresistance to alkalinity, while maintaining melting properties. Thelower limit of the ZrO₂ content is desirably 0.5 percent, preferably 0.7percent, and the upper limit is desirably 3 percent, preferably 2percent, and more preferably, 1.5 percent.

Among the above-described components, SiO₂, Al₂O₃ and ZrO₂ function toenhance chemical durability, and Li₂O, Na₂O and K₂O tend to lowerchemical durability. Accordingly, in Glass II, the upper limit of themolar ratio of the total content of Li₂O, Na₂O and K₂O to the totalcontent of SiO₂, Al₂O₃ and ZrO₂ ((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂)) islimited to equal to or less than 0.28 to maintain glass durability. Thismolar ratio is desirably equal to or less than 0.27, preferably equal toor less than 0.26. The lower limit of this molar ratio is desirably 0.1,preferably 0.15, and more preferably, 0.2.

As needed, clarifying agents such as Sb₂O₃, SnO₂ and CeO₂ may be addedto Glass II. When the glass is being formed by the floating method, theaddition of Sb₂O₃ is undesirable, the addition of SnO₂ and CeO₂ isdesirable, and the addition of SnO₂ is preferred.

[Glass III]

Glass III is glass for use in substrate for information recordingmedium, which comprises SiO₂; Al₂O₃; one or more alkali metal oxidesselected from the group consisting of Li₂O, Na₂O and K₂O; one or morealkaline earth metal oxides selected from the group consisting of MgO,CaO, SrO and BaO; and one or more oxides selected from the groupconsisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

wherein SiO₂ content is equal to or greater than 50 molar percent, andthe total content of SiO₂ and Al₂O₃ is equal to or greater than 70 molarpercent;

the total content of said alkali metal oxides and said alkaline earthmeal oxides is equal to or greater than 8 molar percent; and

the molar ratio of the total content of said oxides to the total contentof said alkali metal oxides and said alkaline earth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO)) isequal to or greater than 0.035.

Glass III can provide a substrate for information recording mediumaffording good resistance to both acidity and alkalinity.

Unless specifically noted otherwise, the individual component contentsand total contents given below in the description of glass III aredenoted as molar percentages, and ratios between contents are given asmolar ratios. Glass III is an oxide glass and the contents of theindividual components are denoted as their values when converted tooxides.

SiO₂, a glass network-forming component, is an essential componentfunctioning to increase the stability, chemical durability, and inparticular, resistance to acidity of the glass; to lower the thermaldiffusion of the substrate; and to raise the heating efficiency of thesubstrate by radiation.

Al₂O₃ also contributes to the formation of the glass network, andfunctions to increase glass stability and chemical durability.

In Glass III, the total content of SiO₂ and Al₂O₃ is equal to or greaterthan 70 percent, preferably equal to or greater than 75 percent, andmore preferably, equal to or greater than 76 percent to increasechemical durability, particularly resistance to acidity. Inconsideration of the melting properties of the glass, the total contentof SiO₂ and Al₂O₃ is desirably equal to or less than 85 percent,preferably equal to or less than 80 percent.

To achieve good glass stability, SiO₂ content is equal to or greaterthan 50 percent, desirably equal to or greater than 60 percent,preferably equal to or greater than 63 percent, and still morepreferably, equal to or greater than 65 percent. However, theincorporation of an excessive quantity of SiO₂ produces unmeltedmaterial in the glass. Thus, the content of SiO₂ is desirably kept toequal to or less than 75 percent, preferably equal to or less than 72percent, and more preferably, equal to or less than 70 percent. Whenglass in which unmelted material is present is processed into asubstrate, portions of the unmelted material are sometimes exposed onthe surface of the substrate, forming protrusions. Substrates havingsuch protrusions cannot be employed as substrates for informationrecording media in which a high degree of smoothness is required.Accordingly, the melting property of a glass employed in a substrate forinformation recording media is an important characteristic.

Al₂O₃ content is desirably equal to or greater than 3 percent,preferably equal to or greater than 5 percent, and more preferably,equal to or greater than 7 percent. However, the incorporation of anexcessive quantity of Al₂O₃ compromises the melting properties of theglass, so the content of Al₂O₃ is desirably equal to or less than 15percent, preferably equal to or less than 12 percent.

As set forth above, Glass III comprises a relatively large total contentof SiO₂ and Al₂O₃. To enhance the melting properties of Glass III, atotal of 8 percent or more of one or more alkali metal oxides selectedfrom the group consisting of Li₂O, Na₂O and K₂O and one or more alkalineearth metal oxides selected from the group consisting of MgO, CaO, SrOand BaO is incorporated. These alkali metal oxides and alkaline earthmetal oxides both enhance the melting properties of the glass and bringthe thermal expansion characteristics to within a range suited to thesubstrate of information recording medium. However, when the totalcontent of alkali metal oxides and alkaline earth metal oxides becomesexcessively high, there is a tendency for chemical durability todecrease. Thus, to maintain chemical durability, it is desirable for thetotal content of Li₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO to be equal toor less than 24 percent. To both improve melting properties and increasethe coefficient of thermal expansion, this total content is desirablyequal to or greater than 10 percent, preferably equal to or greater than15 percent, and more preferably, equal to or greater than 20 percent.

ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ function to enhancechemical durability, particularly resistance to alkalinity. However, theincorporation of excessive quantities compromises the meltingproperties. Accordingly, in Glass III, to achieve both chemicaldurability and melting properties, the total content of these oxides isestablished in relation to the combined quantity of alkaline metaloxides and alkaline earth metal oxides, which improve melting propertiesbut tend to compromise chemical durability.

That is, the molar ratio of the total content of ZrO₂, HfO₂, Nb₂O₅,Ta₂O₅, La₂O₃ and TiO₂ to the total content of Li₂O, Na₂O, K₂O, MgO, CaO,SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035. This makes it possible to enhanceresistance to alkalinity while maintaining melting properties. The molarratio is desirably equal to or greater than 0.040. When the molar ratiobecomes excessively high, the melting properties deteriorate and glassstability tends to decrease. Thus, the molar ratio is desirably equal toor less than 0.18, preferably equal to or less than 0.15, morepreferably equal to or less than 0.13, and still more preferably, equalto or less than 0.12.

To further enhance chemical durability, particularly resistance toalkalinity, the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃and TiO₂ is desirably equal to or greater than 0.3 percent, preferablyequal to or greater than 0.5 percent, and more preferably, equal to orgreater than 0.7. To maintain good melting properties and glassstability, the above-described total content is desirably equal to orless than 4 percent, preferably equal to or less than 3 percent, morepreferably equal to or less than 2 percent, and still more preferably,equal to or less than 1.5 percent.

The details of the contents of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂, and the molar ratio of the ZrO₂ content to the total content ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂(ZrO₂/(ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)) in Glass III are asdescribed above for glass I.

A preferred embodiment of Glass III is glass containing at least one odLi₂O and Na₂O, with the total content of Li₂O and Na₂O being equal to orless than 24 percent. Li₂O and Na₂O are components that increase meltingproperties. Li₂O and Na₂O are important components when chemicallystrengthening Glass III. They also function to strongly impart suitablethermal expansion characteristics to substrates employed in informationrecording media, particularly magnetic recording media, by raising thecoefficient of thermal expansion.

To reduce and prevent the leaching out of alkali metal componentsthrough an effect achieved by mixing alkalis, Li₂O and Na₂O aredesirably incorporated as glass components. However, when excessivelylarge quantities of Li₂O and Na₂O are incorporated, chemical durabilitytends to decrease. Thus, the total content of Li₂O and Na₂O is desirablylimited to equal to or less than 24 percent. To further enhance chemicaldurability and reduce or prevent leaching out of alkali metal ions fromthe glass substrate, the total content of Li₂O and Na₂O is desirablylimited to equal to or less than 22 percent.

The details of the content of Li₂O and Na₂O in Glass III, the molarratio of the quantity of Li₂O to the quantity of Na₂O (Li₂O/Na₂O), andthe content of K₂O are as described above for Glass I.

MgO functions to increase rigidity and hardness and raise thecoefficient of thermal expansion. When present together with CaO, it hasthe effect of increasing the stability of the glass. Since it alsofunctions to reduce the ion exchange rate during chemical strengthening,the incorporation of a suitable quantity can be used to effectivelycontrol the ion exchange rate so that flatness does not decrease.However, incorporation in excessively large quantity compromiseschemical durability. Thus, the content of MgO is desirably equal to orgreater than 0 percent but less than 5 percent. The lower limit of theMgO content is desirably 0.1 percent, preferably 0.3 percent, and morepreferably, 0.5 percent. The MgO content is desirably less than 3percent, preferably equal to or less than 2 percent.

CaO functions to increase rigidity and hardness, raise the coefficientof thermal expansion, and enhance resistance to devitrification whenincorporated in suitable quantity. In the same manner as MgO, CaOfunctions to control the ion exchange rate during chemicalstrengthening. However, introduction in excessively large quantitycompromises chemical durability. Thus, the content of CaO is desirably 0to 5 percent. The lower limit of the CaO content is desirably 0.1percent, preferably 0.5 percent; the upper limit is desirably 4 percent,preferably 3 percent.

As in Glass I, the content of CaO is desirably greater than the contentof MgO in Glass III to further increase resistance to devitrificationand enhance chemical durability. To increase resistance todevitrification and enhance chemical durability, the molar ratio of theMgO content to the CaO content (MgO/CaO) in Glass II, as in Glass I,desirably falls within a range of 0.14 to 0.97, preferably 0.4 to 0.97.

From the above perspectives, the total content of MgO and CaO isdesirably 1 to 6 percent. The lower limit of the total quantity of MgOand CaO is desirably 1.5 percent, preferably 2 percent, and the upperlimit is desirably 5.5 percent, preferably 5 percent, and morepreferably, 4 percent.

Both SrO and BaO function to enhance melting properties and raise thecoefficient of thermal expansion. However, the addition of SrO and BaOtends to compromise chemical durability, increase the specific gravityof the glass, and increase the cost of the starting materials. Thus, thetotal content of SrO and BaO is desirably 0 to 5 percent, preferably 0to 2 percent, and more preferably, 0 to 1 percent. The SrO contentdesirably falls within a range of 0 to 2 percent, preferably 0 to 1percent, with no incorporation of SrO being of even greater preference.The BaO content desirably falls within a range of 0 to 2 percent,preferably 0 to 1 percent, with no incorporation of BaO being of evengreater preference.

To summarize the above perspectives, a preferred embodiment of Glass IIIis glass comprising, denoted in moles, 60 to 75 percent of SiO₂, 3 to 15percent of Al2O₃, 0.3 to 4 percent of ZrO₂; a more preferred embodimentis glass having the aforementioned composition as well as furthercomprising 0.3 to 4 percent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ and TiO₂.

An embodiment of still greater preference is glass in which thequantities of the various alkali metal oxides and alkaline earth metaloxides are allocated as set forth above.

As needed, clarifying agents such as Sb₂O₃, SnO₂ and CeO₂ may be addedto Glass III. When the glass is being formed by the floating method, theaddition of Sb₂O₃ is undesirable, the addition of SnO₂ and CeO₂ isdesirable, and the addition of SnO₂ is preferred.

[Glass IV]

Glass IV is aluminosilicate glass for chemical strengthening for use insubstrate for information recording medium, which comprises:

one or more alkali metal oxides selected from the group consisting ofLi₂O, Na₂O and K₂O, one or more alkaline earth metal oxides selectedfrom the group consisting of MgO, CaO, SrO and BaO, and one or moreoxides selected from the group consisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅,La₂O₃, Y₂O₃ and TiO₂;

wherein the total content of Li₂O and Na₂O is 10 to 22 molar percent;

the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ isgreater than 0 molar percent but equal to or less than 4 molar percent;and

the molar ratio of the total content of said oxides to the total contentof said alkaline earth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(MgO+CaO+SrO+BaO)) is equal toor greater than 0.15.

Glass IV is glass for chemical strengthening, that is, that is subjectedto chemical strengthening. It comprises at least one of Li₂O and Na₂O,preferably both Li₂O and Na₂O, which are necessary for chemicalstrengthening, and is aluminosilicate glass permitting the reduction orprevention of the leaching out of alkali metal ions. Since the totalcontent of Li₂O and Na₂O is limited to reduce or prevent the leachingout of alkalis, one or more alkaline earth metal oxides selected fromthe group consisting of MgO, CaO, SrO and BaO are incorporated toprevent a deterioration in melting properties. However, these alkalineearth metal oxides function to prevent ion exchange during chemicalstrengthening. Further, from the perspective of enhancement ofdeflecting strength by chemical strengthening, the limitation of thetotal content of Li₂O and Na₂O, which undergo ion exchange, functions asa negative. Accordingly, in Glass IV, at least one oxide selected fromthe group consisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ and TiO₂, whichfunction to promote ion exchange and enhance chemical durability,particularly resistance to alkalinity, is incorporated. This providesglass that will undergo good chemical strengthening.

Unless specifically noted otherwise, the individual component contentsand total contents given below in the description of Glass IV aredenoted as molar percentages, and ratios between contents are given asmolar ratios. Glass IV is an oxide glass and the contents of theindividual components are denoted as their values when converted tooxides.

Li₂O and Na₂O are necessary components for ion exchange during chemicalstrengthening and are effective components for improving glass meltingproperties and for achieving a coefficient of thermal expansion fallingwithin a range suited to substrates for use in information recordingmedia, particularly substrates for use in magnetic recording media.

To achieve such effects, the total content of Li₂O and Na₂O is set toequal to or greater than 10 percent, and from the perspective ofreducing or preventing leaching out of alkali metal ions, the abovetotal content is kept to equal to or less than 22 percent. The lowerlimit of the total content of Li₂O and Na₂O is desirably 15 percent,preferably 21 percent. To reduce or prevent the leaching out of alkalimetal components through an effect achieved by mixing alkalis, both Li₂Oand Na₂O are desirably incorporated.

The details of the content of Li₂O and Na₂O in Glass IV, the molar ratioof the quantity of Li₂O to the quantity of Na₂O (Li₂O/Na₂O), and thecontent of K₂O are as described above for Glass I.

ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ promote ion exchange andenhance chemical durability, particularly resistance to alkalinity.However, when incorporated in excessive quantity, they compromisemelting properties and run the risk of generating unmelted material. Asset forth above, when glass containing unmelted material is employed insubstrates for information recording media, particularly substrates formagnetic recording media, portions of the unmelted material, eventhrough extremely small, are sometimes exposed on the surface of thesubstrate, forming protrusions which may impair the smoothness of thesubstrate surface. Accordingly, in Glass IV, the total content of ZrO₂,HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ is set to greater than 0percent but equal to or less than 4 percent. The upper limit of theabove total content is desirably 3 percent, preferably 2 percent, andmore preferably 1.5 percent; the lower limit is desirably 0.3 percent,preferably 0.5 percent, and still more preferably, 0.7 percent.

The details of the content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂, and the molar ratio of the content of ZrO₂ to the total content ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂(ZrO₂/(ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)) in Glass IV are asdescribed for Glass I above.

Glass IV comprises one or more alkaline earth metal oxides selected fromthe group consisting of MgO, CaO, SrO and BaO. These components functionto maintain melting properties and adjust the coefficient of thermalexpansion. Conversely, they also function to impede ion exchange duringchemical strengthening. Accordingly, in Glass IV, a balance is struckbetween the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ andTiO₂, which promote ion exchange during chemical strengthening, and thetotal content of MgO, CaO, SrO, and BaO, thereby permitting goodchemical strengthening. Specifically, the molar ratio of the totalcontent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ and TiO₂ to the total contentof MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(MgO+CaO+SrO+BaO)) is equal toor greater than 0.015.

The total content of MgO, CaO, SrO and BaO desirably falls within arange of greater than 0 percent but equal to or less than 5 percent.When the above content is equal to or less than 5 percent, good chemicalstrengthening can be conducted and the deflecting strength of thesubstrate can be adequately increased. The lower limit of the totalcontent of MgO, CaO, SrO and BaO is desirably 0.1 percent, preferably0.5 percent, more preferably 1 percent, and still more preferably, 1.5percent; the upper limit is desirably 4.5 percent, preferably 4 percent.

SiO₂ is a network-forming component of Glass IV, aluminosilicate glass,and is an essential component functioning to enhance glass stability andchemical durability, particularly resistance to acidity; lower thethermal diffusion of the substrate; and raise the heating efficiency ofthe substrate by radiation.

Al₂O₃ also contributes to glass network formation and functions toenhance glass stability and chemical durability.

In Glass IV, the total content of SiO₂ and Al₂O₃ is desirably equal toor greater than 70 percent to enhance chemical durability, particularlyresistance to acidity, while achieving good glass stability. This totalcontent is desirably equal to or greater than 75 percent, preferablyequal to or greater than 76 percent, to further enhance resistance toacidity. Taking into account the melting properties of the glass, thetotal content of SiO₂ and Al₂O₃ is desirably equal to or less than 85percent, preferably equal to or less than 80 percent.

The content of SiO₂ is desirably equal to or greater than 50 percent,preferably equal to or greater than 60 percent, more preferably equal toor greater than 62 percent, and still more preferably, equal to orgreater than 65 percent to achieve good glass stability. However, whenSiO₂ is incorporated in an excessively large quantity, unmelted materialis generated in the glass. Thus, the quantity of SiO₂ is desirably keptto equal to or less than 75 percent, preferably equal to or less than 72percent, and more preferably, equal to or less than 70 percent. Whenglass in which unmelted material is present is processed into asubstrate, portions of the unmelted material are sometimes exposed onthe surface of the substrate, forming protrusions. Substrates havingsuch protrusions cannot be employed as substrates for informationrecording media in which a high degree of smoothness is required.Accordingly, the melting property of a glass employed in substrates forinformation recording media is an important characteristic.

The content of Al₂O₃ is desirably greater than 0 percent, preferablyequal to or greater than 3 percent, more preferably equal to or greaterthan 5 percent, and more preferably, equal to or greater than 7 percent.However, when Al₂O₃ is incorporated in an excessively large quantity,the melting properties of the glass are compromised. Thus, the contentof Al₂O₃ is desirably kept to equal to or less than 15 percent,preferably equal to or less than 12 percent.

To both achieve chemical durability, particularly resistance toalkalinity, and maintain melting properties, the molar ratio of thetotal content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ to thetotal content of Li₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is desirably equal to or greater than 0.035. The above molar ratio ispreferably equal to or greater than 0.040. When this molar ratio isexcessively high, melting properties deteriorate and the stability ofthe glass tends to decrease. Thus, this molar ratio is desirably equalto or less than 0.18, preferably equal to or less than 0.15, morepreferably equal to or less than 0.13, and still more preferably, equalto or less than 0.12.

MgO functions to enhance melting properties, rigidity, and hardness, andto raise the coefficient of thermal expansion. When present togetherwith CaO, it has the effect of increasing the stability of the glass.However, when incorporated in an excessively large quantity, itcompromises chemical durability. Thus, the content of MgO is desirablyequal to or greater than 0 percent but less than 5 percent. The MgOcontent is preferably less than 3 percent, more preferably equal to orless than 2 percent. The lower limit of the MgO content is desirably 0.1percent, preferably 0.3 percent, and more preferably, 0.5 percent.

CaO functions to enhance melting properties, rigidity, and hardness, andto raise the coefficient of thermal expansion. When incorporated insuitable quantity, it improves resistance to devitrification. However,when incorporated in excessively large quantity, it compromises chemicaldurability. Thus, the content of CaO is desirably 0 to 5 percent. Thelower limit of the CaO content is preferably 0.1 percent, morepreferably 0.5 percent, and the upper limit is preferably 4 percent,more preferably 3 percent.

SrO and BaO both function to enhance melting properties and raise thecoefficient of thermal expansion. However, the addition of SrO and BaOcompromises chemical durability, increases the specific gravity of theglass, and tends to increase the cost of starting materials. Thus, thetotal content of SrO and BaO is desirably 0 to 5 percent, preferably 0to 2 percent, and more preferably, 0 to 1 percent. The content of SrOdesirably falls within a range of 0 to 2 percent, preferably 0 to 1percent, with no incorporation of SrO being of even greater preference.The content of BaO desirably falls within a range of 0 to 2 percent,preferably 0 to 1 percent, with no incorporation of BaO being of evengreater preference.

To summarize the above perspectives, a preferred embodiment of Glass IVis glass comprising, when denoted in moles, equal to or greater than 3percent of Al₂O₃, equal to or greater than 8 percent of a total of Li₂O,Na₂O, K₂O, MgO, CaO, SrO and BaO, and more than 0 percent but equal toor less than 5 percent of a total of MgO, CaO, SrO and BaO. Further,glass in which the quantities of various components in the form ofalkali metal oxides and alkaline earth metal oxides are allocated as setforth above is of even greater preference.

As needed, clarifying agents such as Sb₂O₃, SnO₂ and CeO₂ may be addedto Glass IV. When the glass is being formed by the floating method, theaddition of Sb₂O₃ is undesirable, the addition of SnO₂ and CeO₂ isdesirable, and the addition of SnO₂ is preferred.

[Glass V]

Glass V is glass for use in substrate for information recording medium,which comprises SiO₂; Al₂O₃; one or more alkali metal oxides selectedfrom the group consisting of Li₂O, Na₂O and K₂O; one or more alkalineearth metal oxides selected from the group consisting of MgO, CaO, SrOand BaO; and one or more oxides selected from the group consisting ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

which has an acidity resistance resulting in an etching rate of equal toor less than 3.0 nm/minute when immersed in 0.5 percent (Vol %)hydrogenfluosilicic acid (H₂SiF) aqueous solution maintained at 50° C.;and

which has an alkalinity resistance resulting in an etching rate of equalto or less than 0.1 nm/minute when immersed in 1 mass percent potassiumhydroxide aqueous solution maintained at 50° C.

Glass V has both good resistance to acidity and good resistance toalkalinity. Thus, a substrate in an extremely clean state can beobtained while maintaining good smoothness by constituting a substratefrom Glass V, removing contaminants in the form of organic matter on thesurface of the glass by an acid treatment, and then using an alkalitreatment to prevent adhesion of foreign matter.

Further, the above Glass V has an acidity resistance resulting in anetching rate when immersed in 0.5 percent (Vol %) hydrogenfluosilicicacid (H₂SiF) aqueous solution maintained at 50° C. of equal to or lessthan 3.0 nm/minute, preferably equal to or less than 2.5 nm/minute, morepreferably equal to or less than 2.0 nm/minute, and still morepreferably, equal to or less than 1.8 nm/minute. The above Glass V alsohas an alkalinity resistance resulting in an etching rate of equal to orless than 0.1 nm/minute, preferably equal to or less than 0.09nm/minute, and more preferably, equal to or less tha 0.08 nm/minute whenimmersed in 1 mass percent potassium hydroxide aqueous solutionmaintained at 50° C.

In the present invention, the etching rate is defined as the depth towhich the glass surface is removed per unit of time. For example, in thecase of a glass substrate, this is the depth of the glass substrate thatis removed per unit time. The method of measuring this etching rate isnot specifically limited. The following are examples of such methods.First, the above Glass V is processed into the shape of a (platelike)substrate, a mask treatment is applied to a portion of the glasssubstrate to create a portion that will not be etched, and the glasssubstrate is then immersed in the above hydrogenfluosilicic acid aqueoussolution or potassium hydroxide aqueous solution while in that state.After having been immersed for a unit time, the glass substrate isremoved from the aqueous solution and the difference (etchingdifference) between the masked portion and the unmasked portion iscalculated. On that basis, the amount of etching (etching rate) per unittime is calculated.

Unless specifically noted otherwise, the individual component contentsand total contents given below in the description of Glass V are denotedas molar percentages, and ratios between contents are given as molarratios. Glass V is an oxide glass and the contents of the individualcomponents are denoted as their values when converted to oxides.

A preferred embodiment of Glass V is aluminosilicate glass, that isglass in which SiO₂ content is equal to or greater than 50 percent andthe total content of SiO₂ and Al₂O₃ is equal to or greater than 70percent.

SiO₂, a network-forming component of the above aluminosilicate glass, isan essential component functioning to enhance glass stability andchemical durability, particularly resistance to acidity, as well asreduce the thermal diffusion of the substrate and raise the heatingefficiency of the substrate by radiation.

Al₂O₃ also contributes to glass network formation, and functions toenhance glass stability and chemical durability.

In this glass, the total content of SiO₂ and Al₂O₃ is desirably equal toor greater than 70 percent to enhance chemical durability, particularlyresistance to acidity, while achieving good glass stability. To furtherenhance resistance to acidity, the total content is desirably equal toor greater than 75 percent, preferably equal to or greater than 76percent. Taking melting properties of the glass into consideration, thetotal content of SiO₂ and Al₂O₃ is desirably equal to or less than 85percent, preferably equal to or less than 80 percent.

To both achieve good glass stability and further enhance resistance toacidity, the SiO₂ content is desirably equal to or greater than 50percent, preferably equal to or greater than 55 percent, more preferablyequal to or greater than 60 percent, still more preferably equal to orgreater than 63 percent, and even more preferably, equal to or greaterthan 65 percent. However, the introduction of an excessive quantity ofSiO₂ produces unmelted material in the glass. Thus, the quantity of SiO₂is desirably equal to or less than 75 percent, preferably equal to orless than 72 percent, and more preferably, equal to or less than 70percent. When glass in which unmelted material is present is processedinto a substrate, portions of the unmelted material are sometimesexposed on the surface of the substrate, forming protrusions. Substrateshaving such protrusions cannot be employed as substrates for informationrecording media in which a high degree of smoothness is required.Accordingly, the melting property of glass employed in substrates forinformation recording media is an important characteristic.

Glass V comprises components in the form of one or more alkali metaloxides selected from the group consisting of Li₂O, Na₂O and K₂O; one ormore alkaline earth metal oxides selected from the group consisting ofMgO, CaO, SrO and BaO; and one or more oxides selected from the groupconsisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂. Desirably,the composition is such that the total content of the alkali metaloxides and alkaline earth metal oxides is equal to or greater than 8molar percent, and the molar ratio of the total content of the aboveoxides (ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂) to the totalcontent of the alkali metal oxides and the alkaline earth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035.

In the above aluminosilicate, the total quantity of SiO₂ and Al₂O₃ isrelatively high. Thus, one or more alkali metal oxides selected from thegroup consisting of Li₂O, Na₂O and K₂O and one or more alkaline earthmetal oxides selected from the group consisting of MgO, CaO, SrO and BaOare incorporated to maintain melting properties. Both alkali metaloxides and alkaline earth metal oxides are components that are usefulfor raising the coefficient of thermal expansion to keep it within arange suited to substrates for information recording media, particularlysubstrates for magnetic recording media. However, these components havethe effect of decreasing chemical durability. Thus, one or more oxidesselected from the group consisting of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃,Y₂O₃, and TiO₂, which function to strongly enhance chemical durability,particularly resistance to alkalinity, are incorporated. However, whenZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ are incorporated inexcessively large quantity, the melting properties are compromised andglass stability decreases. Thus, the quantity incorporated is desirablyestablished by achieving a balance with the alkali metal oxides andalkaline earth metal oxides. Specifically, the molar ratio of the totalcontent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ to the totalcontent of Li₂O, Na₂O, K₂O, MgO, CaO, SrO, and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is desirably equal to or greater than 0.035. The above molar ratio ispreferably equal to or greater than 0.040. When the molar ratio becomesexcessively high, the melting properties deteriorate and glass stabilitydecreases. Thus, the molar ratio is desirably equal to or less than0.18, preferably equal to or less than 0.15, more preferably equal to orless than 0.13, and still more preferably, equal to or less than 0.12.

To further enhance chemical durability, particularly resistance toalkalinity, the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃,and TiO₂ is desirably equal to or greater than 0.3 percent, preferablyequal to or greater than 0.5 percent, and more preferably, equal to orgreater than 0.7 percent. To maintain good melting properties and glassstability, the total content is desirably equal to or less than 4percent, preferably equal to or less than 3 percent, more preferablyequal to or less than 2 percent, and still more preferably, equal to orless than 1.5 percent.

Details of the content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, andTiO₂ in Glass V, and the molar ratio of the quantity of ZrO₂ to thetotal quantity of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂(ZrO₂/(ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)) are as described forGlass I.

A preferred embodiment of Glass V is glass containing at least one ofLi₂O and Na₂O, with the total quantity of Li₂O and Na₂O being limited toequal to or less than 24 percent, preferably equal to or less than 22percent. Li₂O and Na₂O are components that enhance melting properties,and are necessary in the chemical strengthening of Glass V. Further,they also strongly function to raise the coefficient of thermalexpansion and impart suitable thermal expansion characteristics tosubstrates for information recording media, particular substrates formagnetic recording media.

To reduce or prevent the leaching out of alkali metal components throughan effect achieved by mixing alkalis, Li₂O and Na₂O are desirablyincorporated as glass components. However, when excessively largequantities of Li₂O and Na₂O are incorporated, chemical durability tendsto decrease. Thus, the total content of Li₂O and Na₂O is desirablylimited to equal to or less than 24 percent, preferably equal to or lessthan 22 percent.

To further enhance chemical durability and reduce or prevent leachingout of alkali metal ions from the glass substrate, the total content ofLi₂O and Na₂O is desirably limited to equal to or less than 22 percent.

Details of the content of Li₂O and Na₂O, the molar ratio of the quantityof Li₂O to the quantity of Na₂O (Li₂O/Na₂O), and the content of K₂O inGlass V are as described above for Glass I.

MgO functions to enhance melting properties, rigidity, and hardness, andto raise the coefficient of thermal expansion. When present togetherwith CaO, it also has the effect of increasing the stability of theglass. Since it also functions to reduce the rate of ion exchange duringchemical strengthening, it can be used to control the rate of ionexchange so that smoothness is not lost when incorporated in suitablequantity. However, it compromises chemical durability when incorporatedin excessively large quantity. Thus, the content of MgO is desirablyequal to or greater than 0 percent but less than 5 percent. The MgOcontent is preferably less than 3 percent, with the upper limitpreferably being 2 percent. The lower limit is desirably 0.1 percent,preferably 0.3 percent, and more preferably, 0.5 percent.

CaO functions to enhance melting properties, rigidity, and hardness, andto raise the coefficient of thermal expansion. When incorporated insuitable quantity, it improves resistance to devitrification. Further,in the same manner as MgO, it functions to control the ion exchange rateduring chemical strengthening. However, when incorporated in excessivelylarge quantity, it compromises chemical durability. Thus, the content ofCaO is desirably 0 to 5 percent. The lower limit of the CaO content isdesirably 0.1 percent, preferably 0.5 percent, and the upper limit isdesirably 4 percent, preferably 3 percent.

SrO and BaO both function to enhance melting properties and raise thecoefficient of thermal expansion. However, the addition of SrO and BaOcompromises chemical durability, increases the specific gravity of theglass, and tends to increase the cost of the starting materials. Thus,the total content of SrO and BaO is desirably 0 to 5 percent, preferably0 to 2 percent, and more preferably, 0 to 1 percent. The content of SrOdesirably falls within a range of 0 to 2 percent, preferably 0 to 1percent, with no incorporation of SrO being of even greater preference.The content of BaO desirably falls within a range of 0 to 2 percent,preferably 0 to 1 percent, with no incorporation of BaO being of evengreater preference.

To summarize the above perspectives, a preferred embodiment of Glass IVis glass comprising, denoted in moles,

60 to 75 percent of SiO₂;

3 to 15 percent of Al₂O₃; and

a total of 0.3 to 4 percent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃,and TiO₂, and further preferred is glass in which the quantities of thevarious components in the form of alkali metal oxides and alkaline earthmetal oxides are allocated as set forth above.

As needed, clarifying agents such as Sb₂O₃, SnO₂, and CeO₂ may be addedto Glass V. When the glass is being formed by the floating method, theaddition of Sb₂O₃ is undesirable, the addition of SnO₂ and CeO₂ isdesirable, and the addition of SnO₂ is preferred.

[Glasses VI, VII]

Glass VI is glass for use in substrate for information recording medium,which comprises, denoted as mass percentages,

57 to 75 percent of SiO₂;

5 to 20 percent of Al₂O₃, with the total content of SiO₂ and Al₂O₃ beingequal to or greater than 74 percent;

a total of greater than 0 percent but equal to or less than 6 percent ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂;

greater than 1 percent but equal to or less than 9 percent of Li₂O;

5 to 18 percent of Na₂O, with the mass ratio, Li₂O/Na₂O being equal toor less than 0.5;

0 to 6 percent of K₂O;

0 to 4 percent of MgO;

greater than 0 percent but equal to or less than 5 percent of CaO, withthe total content of MgO and CaO being equal to or less than 5 percentand CaO content being greater than MgO content; and a total of 0 to 3percent of SrO and BaO.

Glass VII is glass for use in substrate for information recordingmedium, which comprises, denoted as mass percentages,

57 to 75 percent of SiO₂;

5 to 20 percent of Al₂O₃, with the total content of SiO₂ and Al₂O₃ beingequal to or greater than 74 percent;

greater than 0 percent but equal to or less than 5.5 percent of ZrO₂;

greater than 1 percent but equal to or less than 9 percent of Li₂O;

5 to 18 percent of Na₂O, with the mass ratio, Li₂O/Na₂O being equal toor less than 0.5;

0 to 6 percent of K₂O;

0 to 4 percent of MgO;

greater than 0 percent but equal to or less than 5 percent of CaO, withthe total content of MgO and CaO being equal to or less than 5 percentand CaO content being greater than MgO content;

a total of 0 to 3 percent of SrO and BaO; and

0 to 1 percent of TiO₂.

Glasses VI and VII can be used to manufacture substrates for informationrecording media having high chemical durability and good surfacesmoothness following cleaning.

Glass VI defines ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ as thetotal content, and Glass VII defines the content of ZrO₂ and TiO₂respectively; they are otherwise identical. Therefore, the compositionsof Glasses VI and VII will be jointly described in detail below. BothGlasses VI and VII are oxide glasses, and the contents of the variouscomponents are given as their values when converted to oxides. Unlessspecifically noted otherwise, the individual component contents andtotal contents given below in the description of Glasses VI and VII aredenoted as mass percentages, and ratios between contents are given asmass ratios.

SiO₂, a network-forming component, is an essential component thatfunctions to enhance glass stability and chemical durability,particularly resistance to acidity; reduces the thermal diffusion of thesubstrate; and increases the heating efficiency of the substrate byradiation. These effects are difficult to achieve, and the above objectsare difficult to realize, when the content of SiO₂ is less than 57percent. When 75 percent is exceeded, melting properties deteriorate andunmelted material is produced in the glass. Accordingly, the content ofSiO₂ is set to 57 to 75 percent, desirably 63 to 70 percent, andpreferably, 63 to 68 percent.

Al₂O₃ also contributes to glass network formation, functioning toenhance glass stability and chemical durability. The above effects aredifficult to achieve when the content of Al₂O₃ is less than 5 percent;when 20 percent is exceeded, melting properties deteriorate and unmeltedmaterial is produced in the glass. Accordingly, the content of Al₂O₃ is5 to 20 percent, desirably 7 to 20 percent, preferably 11 to 20 percent,more preferably 12 to 20 percent, still more preferably 13 to 20percent, even more preferably 13 to 18 percent, and yet even morepreferably, 13 to 16 percent.

SiO₂ and Al₂O₃ are interchangeable. However, to maintain good glassstability and chemical durability, the total content of SiO₂ and Al₂O₃is set to equal to or greater than 74 percent. This total content isdesirably equal to or greater than 76 percent, preferably equal to orgreater than 78 percent, more preferably, equal to or greater than 79percent, and still more preferably, equal to or greater than 80 percent.

In Glass VI, ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ arecomponents that enhance chemical durability, particularly resistance toalkalinity, and increase rigidity and toughness. Thus, the total contentof ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ is set to greaterthan 0 percent. However, when this total quantity exceeds 6 percent,glass stability drops, melting properties deteriorate, and specificgravity increases. Thus, the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅,La₂O₃, Y₂O₃, and TiO₂ is set to greater than 0 percent but equal to orless than 6 percent. The total quantity is desirably equal to or lessthan 5.5 percent, preferably equal to or less than 4 percent, and morepreferably, equal to or less than 3 percent. The lower limit of theabove content is desirably 0.1 percent, preferably 0.2 percent, morepreferably 0.5 percent, still more preferably 1 percent, and even morepreferably, 1.4 percent.

Among ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂, when glasscontaining TiO₂ is dipped in water, a reaction product of the glass andwater adheres to the glass surface. Thus, the other components areadvantageous with regard to resistance to water. Accordingly, tomaintain water resistance, the content of TiO₂ is desirably 0 to 1percent, preferably 0 to 0.5 percent, with no incorporation of TiO₂being of even greater preference.

HfO₂, Nb₂O₅, Ta₂O₅, and La₂O₃ increase the specific gravity of the glassand the weight of the substrate. Thus, to reduce the weight of thesubstrate, the total content of HfO₂, Nb₂O₅, Ta₂O₅, and La₂O₃ desirablyfalls within a range of 0 to 3 percent, preferably 0 to 2 percent, andmore preferably, 0 to 1 percent, with no incorporation being of evengreater preference. Each of HfO₂, Nb₂O₅, Ta₂O₅, and La₂O₃ is desirablyindividually incorporated in a content of 0 to 3 percent, preferably 0to 2 percent, and more preferably, 0 to 1 percent, with no incorporationbeing of even greater preference.

In order to achieve the above desired effects while maintaining glassstability, the content of Y₂O₃ desirably falls within a range of 0 to 2percent, preferably 0 to 1 percent, with no incorporation of Y₂O₃ beingof even greater preference.

ZrO₂ functions to strongly enhance chemical durability, particularlyresistance to alkalinity; increase rigidity and toughness; and enhancethe efficiency of chemical strengthening. Further, it is less expensiveas a starting material than Y₂O₃. Thus, the mass ratio of the content ofZrO₂ to the total content of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, andTiO₂ desirably falls within a range of 0.8 to 1, preferably 0.9 to 1,more preferably 0.95 to 1, and still more preferably, is 1.

In Glass VII, ZrO₂ is an essential component that functions to enhancechemical durability, particularly resistance to alkalinity; increaserigidity and toughness; and increase the efficiency of chemicalstrengthening, even when incorporated in a small quantity. However, whenan excessively large quantity of ZrO₂ is incorporated into a thinsubstrate, the efficiency of chemical strengthening becomes excessivelyhigh, forming an excessive compression stress layer and tending to causewaviness in the substrate. Accordingly, the content of ZrO₂ is set togreater than 0 percent but equal to or less than 5.5 percent. Thecontent of ZrO₂ desirably falls within a range of 0.1 to 5.5 percent.The lower limit of the ZrO₂ content is desirably 0.2 percent, preferably0.5 percent, more preferably 1 percent; and still more preferably, 1.4percent; the upper limit is desirably 5 percent, preferably 4 percent,and still more preferably, 3 percent.

Alkali metal oxides such as Li₂O, Na₂O, and K₂O function to enhanceglass melting properties and raise the coefficient of thermal expansion,thereby imparting thermal expansion characteristics suited to substratesfor use in information recording media, particularly substrates for usein magnetic recording media. In Glasses VI and VII, among the abovealkali metal oxides, Li₂O and Na₂O are essential components and K₂O isan optional component.

In addition to the above functions, Li₂O is a component that contributesto ion exchange during chemical strengthening, and is incorporated in aquantity of greater than 1 percent. However, it compromises chemicaldurability when incorporated in an excessively large quantity. Thus, thecontent of Li₂O is set to greater than 1 percent but equal to or lessthan 9 percent. The lower limit of Li₂O content is desirably 1.5percent, preferably 2 percent; the upper limit is desirably 7 percent,preferably 5 percent, more preferably 4.5 percent, and still morepreferably, 4.0 percent.

In addition to the above functions, Na₂O is a component that contributesto ion exchange during chemical strengthening, and is incorporated in aquantity of equal to or greater than 5 percent. However, it compromiseschemical durability when incorporated in a quantity of greater than 18percent. Thus, the content is set to 5 to 18 percent. The lower limit ofthe Na₂O content is desirably 6 percent, preferably 7 percent, morepreferably 8 percent, and still more preferably, 9 percent. The upperlimit is desirably 17 percent, preferably 16 percent, and morepreferably, 15 percent.

However, the ratio of the quantity of Li₂O to the quantity of Na₂O(Li₂O/Na₂O) is equal to or less than 0.5, preferably equal to or lessthan 0.45, more preferably equal to or less than 0.4, and still morepreferably, equal to or less than 0.38. Li₂O and Na₂O are glasscomponents that contribute directly to ion exchange during chemicalstrengthening. In molten salt, the alkali ions that contribute to ionexchange are Na ions and/or K ions. As the number of substrates thathave been subjected to chemical strengthening increases, theconcentration of Li ions in the molten salt increases. When largequantities of glass in which (Li₂O/Na₂O) exceeds 0.5 are treated, theconcentration of Li ions in the molten salt increases markedly. Thebalance between the alkali ions contributing to ion exchange and thealkali ions that do not contribute to ion exchange greatly changesrelative to the start of treatment. As a result, the optimal processingconditions existing at the start of treatment move out of the optimalrange as the number of substrates treated increases. As stated above,variation in the shape of the substrate develops, causing an increaseddimensional tolerance in the inner diameter of the center hole. Thereare also problems in that the compression stress layer formsinadequately and waviness develops in the substrate. To resolve suchproblems, Li₂O/Na₂O is set to within the above range. To reduce orprevent the leaching out of alkali metal components through an effectachieved by mixing alkalis, glass comprising both Li₂O and Na₂O isdesirable.

K₂O also functions as the above alkali metal oxide. However, chemicaldurability is compromised when K₂O is incorporated in a quantity ofgreater than 6 percent. Thus, the content of K₂O is set to 0 to 6percent, desirably 0 to 3 percent, preferably 0 to 2 percent, morepreferably 0 to 1 percent, and still more preferably, 0.1 to 0.9percent. The incorporation of a small quantity of K₂O has the effect ofreducing the variation in compression stress layers between substrateswhen chemically strengthening a large number of substrates.

The total content of Li₂O, Na₂O, and K₂O is desirably kept to equal toor less than 24 molar percent, preferably equal to or less than 22 molarpercent, to further enhance chemical durability and reduce the leachingout of alkali metal components from the substrate.

MgO functions to increase rigidity and hardness and increase thecoefficient of thermal expansion. When present together with CaO, it hasthe effect of increasing the stability of the glass. Since it alsofunctions to reduce the ion exchange rate during chemical strengthening,the incorporation of a suitable quantity can be used to effectivelycontrol the ion exchange rate so that the degree of smoothness does notdecrease. However, incorporation in a quantity of greater than 4 percentcompromises chemical durability. Thus, the content of MgO is set to 0 to4 percent. The lower limit of the MgO content is desirably 0.1 percent,preferably 0.2 percent; the upper limit is desirably 3.5 percent.

CaO functions to enhance melting properties, rigidity, and hardness,raise the coefficient of thermal expansion, and enhance resistance todevitrification when incorporated in suitable quantity. In the samemanner as MgO, CaO functions to control the ion exchange rate duringchemical strengthening. However, introduction in excess of 5 percentcompromises chemical durability. Thus, the CaO content is set to greaterthan 0 percent but equal to or less than 5 percent. The lower limit ofthe CaO content is desirably 0.1 percent, preferably 0.3 percent, andmore preferably, 0.5 percent; the upper limit is desirably 4 percent,preferably 3.5 percent.

Chemical durability deteriorates when the total quantity of MgO and CaOexceeds 5 percent. Thus, the total quantity of MgO and CaO is equal toor less than 5 percent, desirably equal to or less than 4.5 percent, andmore preferably, equal to or less than 4 percent. The CaO content is setto greater than the MgO content to achieve good resistance todevitrification. Both MgO and CaO are desirably present as glasscomponents to improve chemical durability. By setting the ratio of thequantity of MgO to the quantity of CaO (MgO/CaO) to 0.1 to 0.9, morepreferably 0.3 to 0.7, it is possible to achieve even higher chemicaldurability and increase glass stability.

SrO and BaO function to enhance melting properties and raise thecoefficient of thermal expansion. However, they decrease chemicaldurability, increase the specific gravity of the glass, and increase thecost of starting materials. Thus, the total content of SrO and BaO is 0to 3 percent, desirably 0 to 2 percent, and preferably 0 to 1 percent,with no incorporation of SrO being of even greater preference. Noincorporation of BaO is also preferred.

In Glass VII, TiO₂ functions to increase rigidity. However, whenintroduced in excessively large quantity, water resistance decreases.Thus, the content is 0 to 1 percent, desirably 0 to 0.5 percent, with noincorporation being preferred. The content of TiO₂ in Glass VI is as setforth above.

ZnO has the same function as the alkaline earth metal oxides, such asenhancing melting properties, but lowers resistance to devitrificationwhen incorporated in excessively large quantity. It is also volatile,and sometimes corrodes refractory materials in melting of the glass.Thus, the content is, for example, less than 1 percent, desirably 0 to0.9 percent, preferably 0 to 0.5 percent, with no incorporation being ofeven greater preference.

B₂O₃ functions to enhance melting properties. However, it is volatile,and sometimes corrodes refractory materials in melting of the glass.Thus, the content is, for example, less than 2 percent, desirably 0 to1.5 percent, preferably 0 to 1 percent, and more preferably 0 to 0.4percent, with no incorporation being of even greater preference.

Gd, Yb, Er, Nd, Dy, Ho, Tm, Tb, Pm, and Pr can be incorporated in GlassVI to increase rigidity and enhance chemical durability. However, theintroduction of an excessively large quantity diminishes resistance todevitrification and increases the specific gravity and cost of startingmaterials. Thus, the total quantity thereof is, for example, less than 2percent, desirably 0 to 1.8 percent, preferably 0 to 1.5 percent, morepreferably 0 to 1 percent, and still more preferably, 0 to 0.8 percent,with no introduction being of even greater preference.

Ln₂O₃ denotes lanthanoid metal oxides, and the content thereof in GlassVII denotes the total content of lanthanoid metal oxides contained inthe glass. Ln₂O₃ functions to increase rigidity and enhance chemicaldurability; they may thus be incorporated for the purpose of increasingrigidity and enhancing chemical durability. Ln denotes, by way ofexample, La, Gd, Y, Yb, Er, Nd, Dy, Ho, Tm, Tb, Pm, and Pr. However, theincorporation of an excessively large quantity decreases resistance todevitrification and increases the specific gravity and cost of startingmaterials. Thus, the content is, for example, less than 2 percent,desirably 0 to 1.8 percent, preferably 0 to 1.5 percent, more preferably0 to 1 percent, and still more preferably, 0 to 0.8 percent, with noincorporation being of even greater preference.

As needed, clarifying agents such as Sb₂O₃, As₂O₃, SnO₂, and CeO₂ may beincorporated. However, As₂O₃ has a negative environmental impact and isthus desirably not employed, particularly when manufacturing substratesthrough the floating method.

Like As₂O₃, the use of Sb₂O₃ should be avoided when manufacturingsubstrates through the floating method. However, it may be employed asan effective clarifying agent when manufacturing molded glass articlesserving as base materials for substrates by press molding or castmolding. The quantity added is, for example, 0 to 1 percent, preferably0 to 0.7 percent.

In contrast to Sb₂O₃ and As₂O₃, both SnO₂ and CeO₂ may be used in themanufacturing of substrates through the floating method. They are addedin a quantity of, for example, 0 to 1.0 percent, preferably 0 to 0.7percent.

The quantities of Al₂O₃ and ZrO₂ added in Glass VII are desirablydetermined in consideration of the following. The components in GlassVII can be roughly divided up into SiO₂; Al₂O₃; ZrO₂; the alkali metaloxides; the alkaline earth metal oxides; and other components. SiO₂ mustbe incorporated in a quantity adequate to enhance chemical durabilityand increase the heating efficiency of the substrate. The alkali metaloxides must be incorporated in quantities adequate to adjust thecoefficient of expansion and enhance melting properties, and forchemical strengthening. The alkaline earth metal oxides must beincorporated in quantities adequate to adjust the coefficient ofexpansion, enhance melting properties, and control the rate at whichchemical strengthening progresses. With respect to the remainingcompounds, Al₂O₃ and ZrO₂, the ratio of the Al₂O₃ content to the ZrO₂content (Al₂O₃/ZrO₂) is desirably equal to or less than 160, preferablyequal to or less than 100, more preferably equal to or less than 50, andstill more preferably, equal to or less than 20 to enhance resistance toalkalinity and resistance to devitrification.

In the embodiments of Glasses I to VII containing SiO₂, the content ofSiO₂ can be kept to equal to or greater than a certain level to decreasethe thermal diffusion of the glass. In information recording media suchas magnetic recording media, a film including an information recordinglayer is formed by sputtering on a substrate in a vacuum chamber. Inthis regard, the heating of the substrate is desirably conducted byradiation. When a substrate is comprised of a glass of lower thermaldiffusion, the heating efficiency can be increased because heat tendsnot diffuse from a substrate that has absorbed infrared radiation andgenerated heat.

Further, in batch-type film forming devices, multiple substrates aresynchronously subjected to a series of steps to form films. Since theheating efficiency is low at the heating position, this step ends uplimiting the throughput of the entire process. Thus, increasing theefficiency with which the substrate is heated is desirable in themanufacturing of information recording media to enhance productivity.

In Glasses I to VII, not only is it efficient to lower the thermaldiffusion of the substrate, but it is also efficient to incorporateadditives that absorb infrared radiation into the glass to increaseinfrared radiation absorption by the glass. Examples of such IRabsorbing additives are: Fe, Cu, Co, Yb, Mn, Nd, Pr, V, Cr, Ni, Mo, Ho,Er, and water. Fe, Cu, Co, Yb, Mn, Nd, Pr, V, Cr, Ni, Mo, Ho, and Er arepresent in the glass as ions. When these ions are reduced, theyprecipitate out into the glass or onto the surface, potentiallycompromising the smoothness of the substrate surface. Thus, theircontent should be kept to 0 to 1 percent, desirably 0 to 0.5 percent,preferably 0 to 0.2 percent. The quantity of Fe incorporated, asconverted to Fe₂O₃, is desirably equal to or less than 1 percent,preferably equal to or less than 0.5 percent, more preferably equal toor less than 0.2 percent, still more preferably equal to or less than0.1 percent, and even more preferably, equal to or less than 0.05percent. The lower limit is desirably 0.01 percent, preferably 0.03percent. A particularly desirable range is 0.03 to 0.02 percent. Whenemploying the above additives, it is desirable to incorporate Fe, whichaffords high IR absorption. In any case, these additives are effectivewhen introduced in extremely small quantities. Thus, glass startingmaterials containing such additives as impurities, such as silicastarting materials, may also be employed. However, the quantity ofimpurities should also be kept to a certain level, so it is necessary tokeep these points in mind when selecting starting materials. Fe formsalloys with the platinum or platinum alloys constituting portions of theglass melt vessel, the stirrers, and the tubes through which the glassflows, damaging the vessel, stirrers, and tubes. Thus, when employingsuch equipment, it is desirable to reduce the quantity of Fe added. Insuch cases, it is preferable not to incorporate Fe₂O₃.

PbO is highly damaging to the environment and increases the specificgravity of the glass, and is thus desirably not incorporated.

Above-described Glasses VI and VII can be basically comprised of glasscomponents in the form of SiO₂, Al₂O₃, ZrO₂, Li₂O, Na₂O, K₂O, MgO, andCaO, and, as needed, clarifying agents. The addition of other componentsincreases the specific gravity, adds to the cost of the startingmaterials, or the like, tending to deviate from the optimal glass. Thus,the total content of SiO₂, Al₂O₃, ZrO₂, Li₂O, Na₂O, K₂O, MgO, and CaOdesirably constitutes equal to or greater than 99 percent of the total,it being preferred for the glass to be essentially entirely comprised ofthese components.

The method of manufacturing Glasses I to VII will be described. First,glass starting materials in the form of oxides, carbonates, nitrates,sulfates, hydroxides, and the like are weighed out so as to yield thedesired composition and mixed to obtain a formulated starting material.This starting material is heated in a refractory furnace; melted at atemperature of 1,400 to 1,600° C., for example; clarified; andhomogenized. In this manner, a homogenous glass melt free of bubbles andunmelted material is obtained. This melt is caused to flow out andformed into a prescribed shape, yielding the above-described glass.

The present invention further relates to chemically strengthened glassfor use in substrate for information recording medium, obtained bysubjecting the glass for information recording medium substrate of thepresent invention to chemical strengthening treatment.

This chemically strengthened glass has the characteristics of the glassfor information recording medium substrate of the present invention asset forth above. Further, as described above, the glass for informationrecording medium substrate of the present invention can avoid theproblem of variation in the shape of the substrate due to the chemicalstrengthening treatment, increased dimensional tolerance of the innerdiameter of the center hole of the substrate. Thus, the chemicallystrengthened glass obtained by chemically strengthening the glass forinformation recording medium substrate of the present invention hassmall dimensional tolerances of the inner diameter of the center holeand is suitable as a substrate for information recording media employedat high recording densities. Further, by providing a chemicallystrengthened layer that has been subjected to a chemical strengtheningtreatment, not only is damage to the substrate effectively preventedduring the steps of manufacturing and shipping of the informationrecording medium, but reliability following assembly into the device canbe also enhanced.

The chemical strengthening of Glasses I to VII is conducted by immersingglass that has been processed into a disk shape, for example, in moltenalkali salt. Examples of molten salts that are suitable for use aresodium nitrate molten salt, potassium nitrate molten salt, and mixturesof these two molten salts. In the chemical strengthening treatment, aglass substrate is contacted with a chemical strengthening treatmentliquid (molten salt) to replace some of the ions contained in the glasssubstrate with larger ions contained in the chemical strengtheningtreatment liquid, thereby chemically strengthening the glass substrate.Once the glass has been immersed in the molten salt, Li ions in thevicinity of the glass surface exchange with Na ions and K ions in themolten salt, and Na ions in the vicinity of the glass surface exchangewith K ions in the molten salt, forming a compression stress layer onthe substrate surface. The temperature of the molten salt duringchemical strengthening is a temperature that is preferably higher thanthe strain point of the glass but lower than the glass transitiontemperature within a temperature range at which the molten salt does notundergo thermal decomposition. Since the molten salt is repeatedlyemployed, the concentration of various alkali ions in the molten saltchanges and trace quantities of glass components other than Li and Nagradually leach out. As a result, the processing conditions drift out ofthe optimal range, as set forth above. This variation in chemicalstrengthening due to such changes over time in the molten salt can bediminished by adjusting the composition of the glass constituting thesubstrate as set forth above. Furthermore, this variation can also bereduced by setting a high concentration of K ions in the molten salt.The fact that a chemical strengthening treatment has been conducted canbe confirmed by observing the glass cross-section (cut surface of thetreated layer) by the Babinet method, by measuring the distribution ofalkali ions (for example, Li⁺, Na⁺, K⁺) in the direction of depth fromthe surface, and the like.

The present invention further relates to a glass substrate forinformation recording medium being comprised of any one of Glasses I toVII above.

Since the glass substrate for in information recording medium of thepresent invention is comprised of one of Glasses I to VII, which havegood chemical durability as set forth above, high surface smoothness canbe maintained following cleaning to remove foreign matter. Further,since the glass substrate for information recording medium of thepresent invention exhibits little variation in the shape of thesubstrate even after chemical strengthening treatment, the dimensionaltolerance of the inner diameter of the center hole can be lowered,rendering the glass substrate suitable for use in high recording densityinformation recording media. The present invention can yield informationrecording media satisfying the current inner diameter dimensionaltolerance specification (±0.025 mm), as well as information recordingmedia capable of meeting the more stringent specification for thedimensional tolerance of the inner diameter of ±0.010 mm.

Further, for example, glass exhibiting little leaching out of alkalimetal components, such as Glass V, can yield a substrate with littleleaching out of alkali due to chemical strengthening and good impactresistance.

A deflecting strength is generally employed as an indicator of theimpact resistance of substrates for information recording media. Basedon the glass for information recording medium substrate of the presentinvention, it is possible to obtain a glass substrate for informationrecording media having a deflecting strength of, for example, equal toor greater than 10 kg, desirably equal to or greater t than 15 kg, andpreferably, equal to or greater than 20 kg. As shown in FIG. 2, thedeflecting strength is determined by placing a steel ball on the centerhole of a substrate that has been positioned on a holder, applying aweight by means of a load cell, and noting the load at which thesubstrate is damaged. The measurement can be conducted with a deflectingstrength measuring and testing device (Shimadzu Autograph DDS-2000), forexample.

Information recording media are in the forms of magnetic recordingmedia, magneto-optical recording media, optical recording media, and thelike, based on the method of recording and reproduction. Among these,the substrate of the present invention is particularly suitable as asubstrate for magnetic recording media, which require high degrees offlatness and smoothness. Magnetic recording media are referred to asmagnetic disks, hard disks, and the like, and are suitable for use inthe internal memory devices (fixed disks, and the like) of desk top PCs,server-use computers, notebook PCs, and mobile PCs; the internal memorydevices of portable recording and reproducing devices that record andreproduce images and/or sound; the recording and reproduction devices ofvehicle audio systems; and the like.

The substrate of the present invention may have a thickness, forexample, of equal to or less than 1.5 mm, desirably equal to or lessthan 1.2 mm, and preferably equal to or less than 1 mm. The lower limitis desirably 0.3 mm. Such a thin substrate tends to develop wavinesswhen subjected to chemical strengthening. However, in the glass of thepresent invention, particularly Glasses VI and VII, the variouscomponents are balanced to achieve a range in which chemicalstrengthening tends not to cause waviness. Thus, a thin substrate ofgood smoothness is obtained following chemical strengthening treatment.Further, the substrate of the present invention can be in the form of adisk with a hole in the center (center hole). Since the variation in theshape of the substrate following chemical strengthening treatment can bereduced, the glass of the present invention can be used to mass producedisk substrates with center holes of low inner diameter dimensionaltolerance.

The present invention further relates to a method of manufacturing aglass substrate for information recording medium, which comprises thesteps of mirror finishing the glass for information recording mediumsubstrate of the present invention, and following mirror polishing,conducting acid cleaning and alkali cleaning. This manufacturing methodis suitable as a method for manufacturing the substrate of the presentinvention. A specific embodiment thereof will be described below.

First, a glass melt is cast into a heat-resistant metal mold. The glassis molded into cylinders and annealed. The lateral surfaces of the glassare then ground by centerless processing or the like. The glass is thensliced to a prescribed thickness to prepare thin disk-shaped substrateblanks.

Alternatively, an outflowing glass melt can be cut to obtain a desiredglass melt gob. The gob is then press molded in a pressing mold toproduce a thin disk-shaped substrate blank.

Still further, a glass melt can be caused to flow into a floating bath,formed into sheet form, annealed and then hollowed into a roundsubstrate blank to prepare a substrate blank.

The substrate blank thus prepared can be subjected to center holeforming, inner and outer circumference processing, lapping, andpolishing to finish it into a disk-shaped substrate. Subsequently, thesubstrate is washed with cleaning agents such as acids and alkalis,rinsed, dried, and, as needed, subjected to the above-described chemicalstrengthening. The chemical strengthening treatment can also beconducted after the mirror polishing step and before the cleaning step.

In this manner, the substrate is exposed to an acid, an alkali, andwater in a series of steps. However, the glass for information recordingmedium substrate of the present invention has good acid resistance,alkali resistance, and water resistance. Thus, the substrate surfacedoes not become rough, and a substrate having a flat, smooth surface isobtained. Details about how the smoothness can be improved as well as asubstrate with little adhering material can be obtained will bedescribed below.

As set forth above, the glass substrate for information recording media(glass substrate for magnetic disks) is subjected to lapping andpolishing to fashion a substrate surface (main surface) shape to serveas the surface on which information will be recorded. However, forexample, during polishing, abrasive polishing grit and adhering materialare present on the main surface immediately after finish polishing(mirror polishing). To remove them, it is necessary to wash the mainsurface after mirror polishing. For example, when conducting chemicalstrengthening after mirror polishing, the chemical strengtheningtreatment ends up changing the shape of the main surface. Further, sincethe strengthening salt adheres to the main surface, cleaning isnecessary. This cleaning may be conducted in the form of acid cleaningand/or alkali cleaning, for example. It is common to employ both. Whensuch is done, when the glass substrate for information recording mediumhas poor resistance to acid and to alkali, the cleaning will roughen thesubstrate surface. Additionally, when the cleaning agent is weakened toprevent roughening of the substrate surface by cleaning, the abrasivepolishing grit, adhering material, and strengthening salt adhering tothe substrate surface are not adequately removed. Accordingly, in orderto reduce the adhering material including abrasive polishing grit aswell as to enhance the smoothness of the substrate surface, it isrequired to impart adequate acid resistance and alkali resistance to theglass substrate for information recording medium.

Recording density has risen progressively in recent years. For example,high recording density information recording media with recordingdensities of equal to or greater than 130 Gbit/inch², preferably equalto or greater than 200 Gbit/inch², are now in demand. Reducing thefloating level of the recording and reproducing head relative to theinformation recording medium is an effective way of achieving highdensity recording. To this end, it is desirable to employ a substratewith a highly flat surface as the substrate in information recordingmedia. For example, a glass substrate for information recording mediahaving a main surface with a surface roughness (Ra) of equal to or lessthan 0.25 nm, preferably equal to or less than 0.2 nm, more preferablyequal to or less than 0.15 nm, is desirable to manufacture informationrecording media with a recording density of equal to or greater than 130Gbit/inch². Achieving this surface roughness makes it possible to reducethe floating level of the recording and reproducing head on theinformation recording medium, achieving a high recording density. In thepresent invention, the phrase “main surface” means the surface on whichthe information recording layer is to be provided or has been provided.Such a surface is the widest surface among the surfaces of theinformation recording medium, and is thus called the “main surface.” Inthe case of a disk-shaped information recording medium, it correspondsto the round surface of the disk (without the center hole when one ispresent).

Any abrasive grit that is capable of achieving a surface roughness ofequal to or less than 0.25 nm, for example, on the main surface of theglass substrate for information recording media may be employed as theabrasive polishing grit used in the above mirror polishing withoutspecific limitation; however, silicon dioxide is desirable. A colloidalsilica, in which the silicon dioxide is in colloidal form, is preferablyemployed in acidic polishing or alkali polishing to prepare the surfaceform of the glass substrate.

In the above-described cleaning, an acid cleaning is suitable forremoving organic material that has adhered to the main substratesurface. In contrast, an alkali cleaning is suitable for removinginorganic material (such as iron) that has adhered to the substratesurface. That is, since the material that is removed differs with acidcleaning and alkali cleaning, it is desirable to employ a combination ofboth when manufacturing a glass substrate for information recordingmedia, and it is preferable to conduct the acid cleaning step and alkalicleaning step sequentially. From the perspective of controlling thecharge of the glass substrate following cleaning, it is desirable toconduct the alkali cleaning after the acid cleaning.

The resistance to acidity and alkalinity required of the above glasssubstrate for information recording media will be described below. Aresistance to acidity such that the etching rate of the above glasssubstrate when immersed in a 0.5 percent (Vol %) hydrogenfluosilicicacid (H₂SiF) aqueous solution maintained at 50° C. is equal to or lessthan 3.0 nm/minute, desirably equal to or less than 2.5 nm/minute,preferably equal to or less than 2.0 nm, and more preferably, equal toor less than 1.8 nm/minute, is desirable, and a resistance to alkalinitysuch that the etching rate when immersed in a 1 mass percent potassiumhydroxide aqueous solution maintained at 50° C. is equal to or less than0.1 nm/minute, preferably equal to or less than 0.09 nm/minute, and morepreferably, equal to or less than 0.08 nm/minute, is desirable.

Since the above glass substrate has high resistance to acidity andalkalinity, it is possible to manufacture a glass substrate with asmooth surface, with less material adhering to the substrate surface.For example, Glass V can be employed as the glass constituting the glasssubstrate.

The present invention further relates to an information recording mediumcomprising an information recording layer on the above glass substratefor information recording medium.

The present invention also relates to a method of manufacturing aninformation recording medium, wherein a glass substrate for informationrecording medium is manufactured by the method of manufacturing a glasssubstrate for information recording medium, and an information recordinglayer is formed on the glass substrate.

The above-described glasses of the present invention permit themanufacturing of substrates of high surface smoothness that afford goodshape stability following chemical strengthening treatment. Theinformation recording medium comprising the above-described substrate issuited to high density recording. Since substrates of the high heatingefficiency can be obtained set forth above, it is also possible tomanufacture information recording media with high productivity.

By suitably selecting the information recording layer, the aboveinformation recording medium can be employed in a variety of informationrecording media. Examples of these media are: magnetic recording media,magneto-optical recording media, and optical recording media.

As set forth above, the information recording medium of the presentinvention can accommodate increasingly high recording densities. Inparticular, it can be suitably employed as a perpendicular magneticrecording-mode magnetic recording medium in. Based on informationrecording media employed in perpendicular magnetic recording systems, itis possible to provide information recording media that can accommodateeven higher recording densities. That is, a magnetic recording medium ina perpendicular magnetic recording system affords a higher recordingdensity (for example, 1 Tbit/(2.5 cm)²) than the surface recordingdensity (equal to or greater than 100 Gbit/(2.5 cm)²) of a magneticrecording medium in a conventional longitudinal magnetic recordingsystem. Thus, even higher density recording can be contemplated.

The information recording medium, and method of manufacturing the same,of the present invention will be described in detail below.

The information recording medium of the present invention comprises aninformation recording layer on the above-described substrate forinformation recording medium. For example, an information recordingmedium such as a magnetic disk can be manufactured by sequentiallyproviding an underlayer, magnetic layer, protective layer, andlubricating layer on the above-described glass substrate.

The information recording layer can be suitably selected based on thetype of medium, and is not specifically limited. Examples are Co—Crbased (here, the word “based” is used to mean a material containing thestated substance), Co—Cr—Pt based, Co—Ni—Cr based, Co—Ni—Pt based,Co—Ni—Cr—Pt based, and Co—Cr—Ta based magnetic layers. An Ni layer, Ni—Player, Cr layer, or the like may be employed as the underlayer. CoCrPtbased alloy materials and, especially, CoCrPtB based alloy materials,are specific examples of materials for use in magnetic layers(information recording layers) suited to high density recording. FePtbased alloy materials are also suitable. These magnetic layers arehighly useful for use as magnetic materials in perpendicular magneticrecording systems. CoCrPt based alloy materials and FePt based alloymaterials can be subjected to film formation or heat treatment followingfilm formation at a high temperature, at 300 to 500° C. for CoCrPt basedalloy materials, and at 500 to 600° C. for FePt alloy materials, toadjust the crystal orientation or crystalline structure and achieveconfigurations suited to high-density recording.

A nonmagnetic underlayer and/or soft magnetic underlayer can be employedas the underlayer. Nonmagnetic underlayers are mainly provided to reducethe size of the crystal particles (crystal grains) in the magnetic layeror control the crystal orientation of the magnetic layer. Underlayerswith bcc system crystallinity, such as Cr based underlayers, havein-plane orientation promoting effects, and are thus desirable inmagnetic disks in in-plane (longitudinal) recording systems. Underlayerswith hcp system crystallinity, such as Ti and Ru based underlayers, haveperpendicular orientation promoting effects, and thus can be employed asmagnetic disks in perpendicular magnetic recording systems. Amorphousunderlayers have the effect of reducing the size of the crystal grainsin the magnetic layer.

Soft magnetic underlayers are employed mainly in perpendicular magneticrecording disks, and have the effect of promoting magnetization patternrecording by magnetic heads in perpendicular magnetic recording layers(magnetic layers). A layer of high saturation magnetic flux density andhigh magnetic permeability is desirable for making full use of theeffects of a soft magnetic underlayer. Thus, high-temperature filmformation or a heat treatment following film formation is desirable.Examples of such soft magnetic layer materials are Fe based softmagnetic materials such as FeTa based soft magnetic materials and FeTaCbased soft magnetic materials. CoZr based soft magnetic materials andCoTaZr based soft magnetic materials are also desirable.

A carbon film or the like may be employed as the protective layer. Alubricating agent such as perfluoropolyether may be employed to form alubricating layer.

An example of a preferred embodiment of a perpendicular magneticrecording disk is a magnetic disk obtained by forming on the substrateof the present invention a soft magnetic underlayer, an amorphousnonmagnetic underlayer, a crystalline nonmagnetic underlayer, aperpendicular magnetic recording layer (magnetic layer), a protectivelayer, and a lubricating layer in this order.

In the case of a perpendicular magnetic recording-mode magneticrecording medium, examples of suitable configurations of the filmsformed on the substrate include: a single layer film in the form of aperpendicular magnetic recording layer formed on a nonmagnetic materialin the form of a glass substrate; a double layer film in the form of asequentially applied soft magnetic layer and magnetic recording layer;and a triple layer film in the form of a sequentially applied hardmagnetic layer, a soft magnetic layer, and a magnetic recording layer.Of these, the double layer and triple layer films are preferred over thesingle layer film because they are better suited to high recordingdensities and maintaining stable magnetic moments.

Based on the glass substrate for information recording media of thepresent invention, it is possible to suitably manufacture magnetic disksfor recording and reproduction at surface information recordingdensities of equal to or greater than 200 Gbits/inch².

Magnetic disks accommodating the perpendicular magnetic recording methodare examples of magnetic disks accommodating surface informationrecording densities of 200 Gbits/inch² and above.

When recording and reproducing information at surface informationrecording densities of 200 Gbits/inch² and above with a hard disk drive,the floating level above the magnetic disk of the magnetic head that isrecording and reproducing signals by floating over the main surface ofthe magnetic disk is equal to or less than 8 nm. To achieve this, themain surface of the magnetic disk is normally a mirror surface. Further,the main surface of the magnetic disk must normally have a surfaceroughness Ra of equal to or less than 0.25 nm. The glass substrate forinformation recording medium of the present invention permits thesuitable manufacturing of a magnetic disk accommodating a magnetic headwith a floating level of equal to or less than 8 nm.

When recording and reproducing information at a surface informationrecording density of equal to or greater than 200 Gbits/inch², therecording and reproducing element that is mounted on the magnetic headis sometimes in the form of a floating level active control elementcalled a dynamic flying height head (“DFH head” hereinafter).

In a DFH head, heating of the area around the element causes the elementportion of the magnetic head to thermally expand, further narrowing thegap between the magnetic head and the magnetic disk. Thus, the mainsurface of the magnetic disk must have a mirror surface with a surfaceroughness Ra of equal to or less than 0.25 nm. The glass substrate forinformation recording medium of the present invention permits thesuitable manufacturing of magnetic disks accommodating the DFH head.

The glass substrate for information recording medium of the presentinvention can be amorphous glass. Amorphous glass can produce mirrorsurfaces of suitable surface roughness.

An implementation embodiment of a magnetic disk that is an informationrecording medium employing the glass substrate for information recordingmedium of the present invention will be described below with referenceto the drawings.

FIG. 1 is an example of the configuration of a magnetic disk 10according to an implementation embodiment of the present invention. Inthe present implementation embodiment, magnetic disk 10 is sequentiallycomprised of glass substrate 12, adhesive layer 14, soft magnetic layer16, underlayer 18, grain size reduction promoting layer 20, magneticrecording layer 22, protective layer 24, and lubricating layer 26.

Magnetic recording layer 22 functions as an information recording layerfor the recording and reproducing of information.

Magnetic disk 10 may be further provided with an amorphous seed layerbetween soft magnetic layer 16 and underlayer 18. The seed layer is alayer for enhancing the crystal orientation of underlayer 18. Forexample, when underlayer 18 is Ru, the seed layer can be a layer forenhancing the C-axis orientation of an hcp crystal structure.

Glass substrate 12 is a glass substrate for the formation of the variouslayers of magnetic disk 10. The above-described glass substrate forinformation recording medium of the present invention is employed asthis glass substrate.

The main surface of the glass substrate is desirably a mirror surfacewith a surface roughness Ra of equal to or less than 0.25 nm. A mirrorsurface with a surface roughness Rmax of equal to or less than 3 nm isdesirable.

Employing such a smooth mirror surface makes it possible to achieve acertain spacing between magnetic recording layer 22, which is aperpendicular magnetic recording layer, and soft magnetic layer 16.Thus, suitable magnetic paths can be formed between the head, magneticrecording layer 22, and soft magnetic layer 16.

Adhesive layer 14 is a layer for increasing the adhesion between glasssubstrate 12 and soft magnetic layer 16, that is formed between glasssubstrate 12 and soft magnetic layer 16. Using adhesive layer 14 canprevent the separation of soft magnetic layer 16. A Ti-containingmaterial, for example, can be employed as the material of adhesive layer14. From the practical perspectives, the film thickness of adhesivelayer 14 is desirably from 1 nm to 50 nm. The material of adhesive layer14 is desirably an amorphous material.

Soft magnetic layer 16 is a layer for adjusting the magnetic circuit ofmagnetic recording layer 22. Soft magnetic layer 16 is not specificallylimited other than that it be formed of a magnetic material exhibitingsoft magnetic characteristics. For example, it desirably exhibitsmagnetic characteristics such as a coercivity (Hc) of 0.01 to 80 Oe,preferably 0.01 to 50 Oe, and a saturation magnetic flux density (Bs) of500 emu/cc to 1920 emu/cc. Examples of the material of soft magneticlayer 16 are Fe based and Co based materials. Specific examples are: Febased soft magnetic materials such as FeTaC based alloys, FeTaN basedalloys, FeNi based alloys, FeCoB based alloys, and FeCo based alloys; Cobased soft magnetic materials such as CoTaZr based alloys and CoNbZrbased alloys; and FeCo based alloy soft magnetic materials. The materialof soft magnetic layer 16 is desirably an amorphous material.

The thickness of soft magnetic layer 16 is, for example, 30 nm to 1,000nm, preferably 50 nm to 200 nm. At less than 30 nm, it sometimes becomesdifficult to form a suitable magnetic circuit between the head, magneticrecording layer 22, and soft magnetic layer 16. At greater than 1,000nm, surface roughness sometimes increases. Further, at greater than1,000 nm, film formation by sputtering is sometimes rendered difficult.

Underlayer 18 is a layer for controlling the crystal orientation ofgrain size reduction promoting layer 20 and magnetic recording layer 22,and may contain ruthenium (Ru), for example. In the presentimplementation embodiment, underlayer 18 is comprised of multiplelayers. In underlayer 18, the layer including the surface interfacingwith grain size reduction promoting layer 20 is formed of Ru crystalgrains.

Grain size reduction promoting layer 20 is a nonmagnetic layer ofgranular structure. In the present implementation embodiment, grain sizereduction promoting layer 20 is comprised of a nonmagnetic CoCrSiOmaterial of granular structure. Grain size reduction promoting layer 20has a granular structure comprised of SiO-containing oxide grainboundary portions and CoCr-containing metal grain portions divided intosections by the grain boundary portions.

Magnetic recording layer 22, ferromagnetic layer 32, magnetic couplingcontrol layer 34, and exchange energy control layer 36 are present ongrain size reduction promoting layer 20 in the order given.Ferromagnetic layer 32 is a CoCrPtSio layer with a granular structure,and CoCrPt crystal grains are present as magnetic crystal grains.

Ferromagnetic layer 32 has a granular structure comprised ofSiO-containing oxide grain boundary portions and CoCrPt-containing metalgrain portions divided into sections by the grain boundary portions.

Magnetic coupling control layer 34 is a coupling control layer forcontrolling magnetic coupling of ferromagnetic layer 32 and exchangeenergy control layer 36. Magnetic coupling control layer 34 is comprisedof a palladium (Pd) layer or a platinum (Pt) layer, for example.Magnetic coupling control layer 34 is equal to or less than 2 nm,preferably 0.5 to 1.5 nm, in thickness, for example.

Exchange energy control layer 36 is a magnetic layer (continuous layer)the easily magnetized axis of which is roughly aligned in the samedirection as ferromagnetic layer 32. Through exchange coupling withferromagnetic layer 32, exchange energy control layer 36 enhances themagnetic recording properties of magnetic disk 10. Exchange energycontrol layer 36 is comprised, for example, of multiple films in theform of alternating stacked layers of cobalt (Co), or an alloy thereof,and palladium (Pd) ([CoX/Pd]n), or alternating stacked layers of cobalt(Co), or an alloy thereof, and platinum (Pt) [CoX/Pt]n), and isdesirably 1 to 8 nm, preferably 3 to 6 nm, in thickness.

Protective film 24 is a protective film for protecting magneticrecording layer 22 from impact by the magnetic head. Lubricating layer26 is a layer for increasing lubrication between magnetic disk 10 andthe magnetic head.

With the exception of lubricating layer 26 and protective layer 24, allof the layers of magnetic disk 10 are desirably formed by sputtering.The use of DC magnetron sputtering yields uniform films and is thusparticularly desirable.

By way of example, protective film 24 can be formed by CVD method usinga hydrocarbon as the material gas, and lubricating layer 26 can beformed by dipping method.

In the present implementation embodiment, an amorphous layer (forexample, adhesive layer 14) is desirably formed by contact with anamorphous glass substrate having a mirror surface. In addition, softmagnetic layer 16 is suitably made of amorphous materials. According tothe present invention, the surface roughness of a glass substrate with amirror surface having an Ra of equal to or less than 0.25 nm, forexample, is reflected on achieving a magnetic disk surface having amirror surface with an Ra of equal to or less than 0.25 nm, for example.

The size of the substrate (for example, magnetic disk substrate) forinformation recording medium and the information recording medium (suchas magnetic disks) of the present invention is not specifically limited.Since high density recording is possible, the size of both the mediumand the substrate can be reduced. For example, they are suited asmagnetic disk substrates and magnetic disks with a nominal diameter of2.5 inches, or even smaller (for example, 1 inch).

EXAMPLES

The present invention will be described in greater detail below throughExamples. However, the present invention is not limited to theembodiments described in Examples.

(1) Preparation of glass melt

Starting materials in the form of oxides, carbonates, nitrates,hydroxides, and the like were weighed out to yield glasses of thecompositions of Examples 1, 1′, 2, 2′, 3, 3′, and 4 to 15 shown in Table1 and mixed to prepare formulated starting materials. These startingmaterials were charged to melt vessels, heated for 6 hours at 1,400 to1,600° C., heated, melted, clarified, and stirred to prepare homogenousglass melts containing no bubbles or unmelted material. Examples 1 to 4,1′ to 3′, and 7 to 15 correspond to glasses I to IV, VI, and VII,Example 5 corresponds to glasses III, IV, VI, and VII; and Example 6corresponds to glasses I to IV.

(2) Glass Molding

The glass melt was then caused to flow out of a pipe at a certain flowrate while being received by the lower mold for press molding, and acutting blade was used to cut the glass melt to obtain a glass melt gobof prescribed weight on the lower mold. The lower mold upon which theglass melt gob had been positioned was immediately conveyed away frombeneath the pipe. Employing an upper mold opposing the lower mold and asleeve, the glass melt gob was press molded into a thin disk shapemeasuring 66 mm in diameter and 1.2 mm in thickness. The press-moldedarticle was cooled to a temperature at which it would not deform,removed from the mold, and annealed, yielding a substrate blank. Theabove molding was conducted with multiple lower molds to continuouslymold the outflowing glass melt.

(3) Preparation of Substrate Blanks

Disk-shaped substrate blanks were prepared by methods A or B below.

(Method A)

The above glass melt was continuously cast from above into thethrough-holes of a heat-resistant casting mold equipped with cylindricalthrough-holes, formed into cylinders, and removed from the bottoms ofthe through holes. The glass that was removed was annealed and thensliced at a prescribed spacing in a direction perpendicular to the axisof the cylinders with a multiwire saw to prepare disk-shaped substrateblanks.

(Method B)

The above glass melt was caused to flow out onto a float bath to form asheet-shaped glass. The sheet glass was then annealed and disk-shapedpieces of glass were cut out of the sheet glass to obtain substrateblanks.

(4) Preparation of Substrates

Through-holes were formed in the center of the substrate blanks obtainedby the above methods, the inner and outer perimeters of the substrateblanks were ground, and the main surfaces of the disks were lapped andpolished (mirror polished) to finish magnetic disk substrates measuring65 mm in diameter and 0.7 mm in thickness.

(5) Cleaning

Next, the substrates were washed with cleaning agents such as acids andalkalis, rinsed with pure water, and dried. The surfaces of thesesubstrates were observed under magnification, revealing no surfaceroughness; they exhibited smooth surfaces.

(5) Chemical Strengthening Treatment

The dried substrates were chemically strengthened by being immersed for240 minutes in a mixed molten salt of sodium nitrate and potassiumnitrate heated to 380° C., washed, and dried. The chemicallystrengthened substrates exhibited no waviness caused by the chemicalstrengthening and a high degree of flatness. The inner diameterdimension of the center hole of the disk-shaped glass substrates waskept within the range of 20.025 mm±0.010 mm, a tolerance that wastighter than the current inner diameter dimension tolerancespecification (tolerance±0.025 mm).

(6) Preparation of Magnetic Disks

An underlayer, soft magnetic layer, magnetic layer, lubricating layerand the like were formed on the chemically strengthened substrates toprepare magnetic disks for a perpendicular magnetic recording system.

Evaluation Method

1. Liquidus Temperature

A glass sample was charged to a platinum crucible, maintained for threehours at a prescribed temperature, removed from the furnace, and cooled.The presence, or absence, of crystal precipitation was then observed bymicroscopy, and the lowest temperature at which no crystals wereobserved was adopted as the liquidus temperature (L.T.). The results aregiven in Table 1.

The liquidus temperature is an indicator of glass stability andresistance to devitrification. A desirable liquidus temperature forglass employed in the substrates of information recording media is equalto or lower than 1,000° C., preferably equal to or lower than 970° C.,more preferably equal to or lower than 950° C., and still morepreferably, equal to or lower than 930° C. The lower limit is notspecifically limited, but 800° C. or above may be employed as ayardstick.

2. Acid Etching Rate and Alkali Etching Rate

Substrates were prepared from the glasses of Examples 1 to 4 by the samemethods as in (1) to (4) above. One portion of each substrate preparedwas subjected to a masking treatment to prevent etching. The glasssubstrate in this state was then immersed for a prescribed period in a0.5 volume percent hydrogenfluosilicic acid aqueous solution maintainedat 50° C. or a 1 mass percent potassium hydroxide aqueous solutionmaintained at 50° C. Subsequently, the glass substrate was withdrawnfrom the above aqueous solution. The difference (etching difference)between the masked and unmasked portions was determined and divided bythe immersion time to calculate the amount of etching (etching rate) perunit time. The results are given in Table 2.

TABLE 1 Denoted as molar percentages Component Example 1 Example 1′Example 2 Example 2′ SiO₂ 66.23 66.29 67.34 67.39 Al₂O₃ 9.25 9.25 9.229.21 Li₂O 8.13 8.12 7.52 7.52 Na₂O 11.22 11.21 10.74 10.74 K₂O 0.26 0.260.26 0.26 MgO 1.54 1.54 1.54 1.54 CaO 2.32 2.32 2.33 2.33 SrO 0.00 0.000.00 0.00 BaO 0.00 0.00 0.00 0.00 ZrO₂ 1.01 1.01 1.01 1.01 HfO₂ 0.000.00 0.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 Ta₂O₅ 0.00 0.00 0.00 0.00 La₂O₃0.00 0.00 0.00 0.00 Y₂O₃ 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00Fe₂O₃ 0.04 0.00 0.04 0.00 ZnO 0.00 0.00 0.00 0.00 Sb₂O₃ 0.00 0.00 0.000.00 SO₃ 0.00 0.00 0.00 0.00 Total 100.00 100.00 100.00 100.00 SiO₂ +Al₂O₃ 75.48 75.54 76.56 76.6 Li₂O + Na₂O + K₂O 19.61 19.59 18.52 18.52(═R₂O) Li₂O + Na₂O 19.35 19.33 18.26 18.26 Li₂O/Na₂O 0.725 0.724 0.7000.700 MgO + CaO + SrO + BaO 3.86 3.86 3.87 3.87 (═RO) CaO + MgO 3.863.86 3.87 3.87 MgO/CaO 0.664 0.664 0.661 0.661 R₂O + RO 23.47 23.4522.39 22.39 ZrO₂ + HfO₂ + Nb₂O₅ + 1.010 1.010 1.010 1.010 Ta₂O₅ +La₂O₃ + Y₂O₃ + TiO₂(=M) R₂O/(SiO₂ + Al₂O₃ + M) 0.256 0.256 0.239 0.239ZrO₂/M 1.000 1.000 1.000 1.000 M/(R₂O + RO) 0.043 0.043 0.045 0.045 M/RO0.262 0.262 0.261 0.261 Liquidus temp. (° C.) 930 930 950 950Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV, VI, VII I~IV, VI,VII Component Example 3 Example 3′ Example 4 Example 5 SiO₂ 67.26 67.3367.3 72.2 Al₂O₃ 9.21 9.2 9.2 6.0 Li₂O 8.14 8.13 6.3 8.2 Na₂O 11.23 11.2312.1 10.4 K₂O 0.26 0.26 0.3 0.1 MgO 1.08 1.08 1.6 0.8 CaO 1.77 1.77 2.20.8 SrO 0.00 0.00 0.0 0.0 BaO 0.00 0.00 0.0 0.0 ZrO₂ 1.01 1.00 1.0 1.5HfO₂ 0.00 0.00 0.0 0.0 Nb₂O₅ 0.00 0.00 0.0 0.0 Ta₂O₅ 0.00 0.00 0.0 0.0La₂O₃ 0.00 0.00 0.0 0.0 Y₂O₃ 0.00 0.00 0.0 0.0 TiO₂ 0.00 0.00 0.0 0.0Fe₂O₃ 0.04 0.00 0.0 0.0 ZnO 0.00 0.00 0.0 0.0 Sb₂O₃ 0.00 0.00 0.0 0.0SO₃ 0.00 0.00 0.0 0.0 Total 100.00 100.00 100.0 100.0 SiO₂ + Al₂O₃ 76.4776.53 76.5 78.2 Li₂O + Na₂O + K₂O 19.63 19.62 18.7 18.7 (═R₂O) Li₂O +Na₂O 19.37 19.36 18.4 18.6 Li₂O/Na₂O 0.725 0.724 0.521 0.788 MgO + CaO +SrO + BaO 2.85 2.85 3.8 1.6 (═RO) CaO + MgO 2.85 2.85 3.8 1.6 MgO/CaO0.610 0.610 0.727 1.000 R₂O + RO 22.48 22.47 22.5 20.3 ZrO₂ + HfO₂ +Nb₂O₅ + 1.010 1.000 1.000 1.500 Ta₂O₅ + La₂O₃ + Y₂O₃ + TiO₂(=M)R₂O/(SiO₂ + Al₂O₃ + M) 0.253 0.253 0.241 0.235 ZrO₂/M 1.000 1.000 1.0001.000 M/(R₂O + RO) 0.045 0.045 0.044 0.074 M/RO 0.354 0.351 0.263 0.938Liquidus temp. (° C.) 920 920 930 950 Corresponding glass I~IV, VI, VIII~III, V, VI I~III, V, VI III, IV, VI, VII Component Example 6 Example 7Example 8 SiO₂ 61.1 63.1 72.0 Al₂O₃ 15.0 13.2 6.0 Li₂O 7.1 8.5 8.2 Na₂O11.4 9.2 10.4 K₂O 1.0 0.9 0.0 MgO 1.4 0.5 0.0 CaO 2.0 2.2 2.5 SrO 0.00.0 0.0 BaO 0.0 0.0 0.0 ZrO₂ 1.0 1.5 0.9 HfO₂ 0.0 0.0 0.0 Nb₂O₅ 0.0 0.90.0 Ta₂O₅ 0.0 0.0 0.0 La₂O₃ 0.0 0.0 0.0 Y₂O₃ 0.0 0.0 0.0 TiO₂ 0.0 0.00.0 Fe₂O₃ 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0Total 100.0 100.0 100.0 SiO₂ + Al₂O₃ 76.1 76.3 78 Li₂O + Na₂O + K₂O 18.517.7 18.6 (═R₂O) Li₂O + Na₂O 19.5 18.6 18.6 Li₂O/Na₂O 0.623 0.924 0.788MgO + CaO + SrO + BaO 3.4 2.7 2.5 (═RO) CaO + MgO 3.4 2.7 2.5 MgO/CaO0.700 0.227 0.000 R₂O + RO 21.9 20.4 21.1 ZrO₂ + HfO₂ + Nb₂O₅ + 1.0002.400 0.900 Ta₂O₅ + La₂O₃ + Y₂O₃ + TiO₂(=M) R₂O/(SiO₂ + Al₂O₃ + M) 0.2400.225 0.236 ZrO₂/M 1.000 0.625 1.000 M/(R₂O + RO) 0.046 0.118 0.043 M/RO0.294 0.889 0.360 Liquidus temp. (° C.) 930 950 970 Corresponding glassI~IV I~IV, VI, VII I~IV, VI, VII Component Example 1 Example 1′ Example2 Example 2′ SiO₂ 64.1 64.2 65.0 65.1 Al₂O₃ 15.2 15.2 15.1 15.1 Li₂O 3.93.9 3.6 3.6 Na₂O 11.2 11.2 10.7 10.7 K₂O 0.4 0.4 0.4 0.4 MgO 1.0 1.0 1.01.0 CaO 2.1 2.1 2.1 2.1 SrO 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 ZrO₂ 2.02.0 2.0 2.0 HfO₂ 0.0 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 0.0 Ta₂O₅ 0.0 0.0 0.00.0 La₂O₃ 0.0 0.0 0.0 0.0 Y₂O₃ 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0Fe₂O₃ 0.1 0.0 0.1 0.0 ZnO 0.0 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.0 0.0 SO₃ 0.00.0 0.0 0.0 Total 100.0 100.0 100.0 100.0 SiO₂ + Al₂O₃ 79.3 79.4 80.180.2 Li₂O/Na₂O 0.348 0.348 0.336 0.336 Li₂O + Na₂O 15.1 15.1 14.3 14.3Li₂O + Na₂O + K₂O 15.5 15.5 14.7 14.7 MgO + CaO 3.1 3.1 3.1 3.1 MgO/CaO0.476 0.476 0.476 0.476 SrO + BaO 0.0 0.0 0.0 0.0 Liquidus temp. (° C.)930 930 950 950 Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV,VI, VII I~IV, VI, VII Component Example 3 Example 3′ Example 4 Example 5SiO₂ 65.0 65.1 64.6 71.0 Al₂O₃ 15.1 15.1 15.0 10.0 Li₂O 3.9 3.9 3.0 4.0Na₂O 11.2 11.2 12.0 10.6 K₂O 0.4 0.4 0.5 0.2 MgO 0.7 0.7 1.0 0.5 CaO 1.61.6 2.0 0.7 SrO 0.0 0.0 0.0 0.0 BaO 0.0 0.0 0.0 0.0 ZrO₂ 2.0 2.0 2.0 3.0HfO₂ 0.0 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 0.0 Ta₂O₅ 0.0 0.0 0.0 0.0 La₂O₃0.0 0.0 0.0 0.0 Y₂O₃ 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 0.0 Fe₂O₃ 0.1 0.00.0 0.0 ZnO 0.0 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0 0.0Total 100.0 100.0 100.0 100.0 SiO₂ + Al₂O₃ 80.1 80.2 79.6 81.0 Li₂O/Na₂O0.348 0.348 0.250 0.377 Li₂O + Na₂O 15.1 15.1 15.0 14.6 Li₂O + Na₂O +K₂O 15.5 15.5 15.4 14.8 MgO + CaO 2.3 2.3 3.0 1.2 MgO/CaO 0.438 0.4380.522 0.714 SrO + BaO 0.0 0.0 0.0 0.0 Liquidus temp. (° C.) 920 920 930950 Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV, VI, VII III,IV, VI, VII Component Example 6 Example 7 Example 8 SiO₂ 56.3 57.3 71.2Al₂O₃ 23.5 20.4 10.1 Li₂O 3.3 3.8 4.0 Na₂O 10.9 8.6 10.6 K₂O 1.5 1.3 0.0MgO 0.9 0.3 0.0 CaO 1.7 1.9 2.3 SrO 0.0 0.0 0.0 BaO 0.0 0.0 0.0 ZrO₂ 1.92.8 1.8 HfO₂ 0.0 0.0 0.0 Nb₂O₅ 0.0 3.6 0.0 Ta₂O₅ 0.0 0.0 0.0 La₂O₃ 0.00.0 0.0 Y₂O₃ 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.0 0.0 ZnO 0.0 0.00.0 Sb₂O₃ 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0 Total 100.0 100.0 100.0 SiO₂ +Al₂O₃ 79.8 77.7 81.3 Li₂O/Na₂O 0.303 0.442 0.377 Li₂O + Na₂O 14.2 12.414.6 Li₂O + Na₂O + K₂O 15.7 13.7 14.6 MgO + CaO 2.6 2.2 2.3 MgO/CaO0.529 0.158 0.000 SrO + BaO 0.0 0.0 0.0 Liquidus temp. (° C.) 930 950970 Corresponding glass I~IV I~IV, VI, VII I~IV, VI, VII ComponentExample 9 Example 10 Example 11 SiO₂ 66.9 68.1 64.8 Al₂O₃ 10.5 10.0 12.0Li₂O 8.3 10.0 5.3 Na₂O 12.0 10.0 14.2 K₂O 0.0 0.0 0.1 MgO 0.0 0.0 0.3CaO 1.3 1.0 2.3 SrO 0.0 0.0 0.0 BaO 0.0 0.0 0.0 ZrO₂ 1.0 0.9 1.0 HfO₂0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 Ta₂O₅ 0.0 0.0 0.0 La₂O₃ 0.0 0.0 0.0 Y₂O₃0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.0 0.0 ZnO 0.0 0.0 0.0 Sb₂O₃ 0.00.0 0.0 SO₃ 0.0 0.0 0.0 Total 100.0 100.0 100.0 SiO₂ + Al₂O₃ 77.4 78.176.8 Li₂O + Na₂O + K₂O 20.3 20.0 19.5 (═R₂O) Li₂O + Na₂O 20.3 20 19.6Li₂O/Na₂O 0.692 1.000 0.373 MgO + CaO + SrO + BaO 1.3 1 2.6 (═RO) CaO +MgO 1.3 1 2.6 MgO/CaO 0.000 0.000 0.130 R₂O + RO 21.6 21.0 22.1 ZrO₂ +HfO₂ + Nb₂O₅ + 1.000 0.900 1.000 Ta₂O₅ + La₂O₃ + Y₂O₃ + TiO₂(=M)R₂O/(SiO₂ + Al₂O₃ + M) 0.259 0.253 0.251 ZrO₂/M 1.000 1.000 1.000M/(R₂O + RO) 0.046 0.043 0.045 M/RO 0.769 0.900 0.385 Liquidus temp. (°C.) 950 950 950 Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV,VI, VII Component Example 12 Example 13 Example 14 SiO₂ 65.3 65.1 68.0Al₂O₃ 11.2 12.3 10.3 Li₂O 4.3 8.3 8.0 Na₂O 14.5 9.5 9.5 K₂O 0.1 0.3 0.2MgO 1.1 1.0 1.0 CaO 2.0 1.7 1.6 SrO 0.0 0.2 0.0 BaO 0.0 0.2 0.0 ZrO₂ 1.51.3 1.4 HfO₂ 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 Ta₂O₅ 0.0 0.1 0.0 La₂O₃ 0.00.0 0.0 Y₂O₃ 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.0 0.0 ZnO 0.0 0.00.0 Sb₂O₃ 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0 Total 100.0 100.0 100.0 SiO₂ +Al₂O₃ 76.5 77.4 78.3 Li₂O + Na₂O + K₂O 18.8 17.8 17.5 (═R₂O) Li₂O + Na₂O18.9 18.1 17.7 Li₂O/Na₂O 0.297 0.874 0.842 MgO + CaO + SrO + BaO 3.1 3.12.6 (═RO) CaO + MgO 3.1 2.7 2.6 MgO/CaO 0.550 0.588 0.625 R₂O + RO 21.920.9 20.1 ZrO₂ + HfO₂ + Nb₂O₅ + 1.500 1.400 1.400 Ta₂O₅ + La₂O₃ + Y₂O₃ +TiO₂(=M) R₂O/(SiO₂ + Al₂O₃ + M) 0.241 0.226 0.220 ZrO₂/M 1.000 0.9291.000 M/(R₂O + RO) 0.068 0.067 0.070 M/RO 0.484 0.452 0.538 Liquidustemp. (° C.) 930 940 960 Corresponding glass I~IV, VI, VII I~IV, VI, VIII~IV, VI, VII Component Example 15 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3SiO₂ 69.0 66.8 67.8 66.3 Al₂O₃ 7.0 10.5 9.1 10.2 Li₂O 8.0 8.6 7.3 10.2Na₂O 11.5 9.3 9.0 8.3 K₂O 0.1 0.3 0.0 0.0 MgO 1.0 0.9 3.8 4.5 CaO 2.02.5 2.2 0.0 SrO 0.4 0.6 0.0 0.0 BaO 0.0 0.2 0.0 0.0 ZrO₂ 0.9 0.0 0.8 0.5HfO₂ 0.1 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 0.0 Ta₂O₅ 0.0 0.0 0.0 0.0 La₂O₃0.0 0.0 0.0 0.0 Y₂O₃ 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.1 0.0 0.0 Fe₂O₃ 0.0 0.10.0 0.0 ZnO 0.0 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.0 0.0 SO₃ 0.0 0.1 0.0 0.0Total 100.0 100.0 100.0 100.0 SiO₂ + Al₂O₃ 76 77.3 76.9 76.5 Li₂O +Na₂O + K₂O 19.5 17.9 16.3 18.5 (═R₂O) Li₂O + Na₂O 19.6 18.2 16.3 18.5Li₂O/Na₂O 0.696 0.925 0.811 1.229 MgO + CaO + SrO + BaO 3.4 4.2 6 4.5(═RO) CaO + MgO 3 3.4 6 4.5 MgO/CaO 0.500 0.360 1.727 — R₂O + RO 22.922.1 22.3 23 ZrO₂ + HfO₂ + Nb₂O₅ + 1.000 0.100 0.800 0.500 Ta₂O₅ +La₂O₃ + Y₂O₃ + TiO₂(=M) R₂O/(SiO₂ + Al₂O₃ + M) 0.253 0.231 0.210 0.240ZrO₂/M 0.900 0.000 1.000 1.000 M/(R₂O + RO) 0.044 0.005 0.036 0.022 M/RO0.294 0.024 0.133 0.111 Liquidus temp. (° C.) 940 Corresponding glassI~IV, VI, VII — — — Component Example 9 Example 10 Example 11 SiO₂ 63.966.0 60.5 Al₂O₃ 17.1 16.5 19.0 Li₂O 3.9 4.8 2.5 Na₂O 11.9 10.0 13.7 K₂O0.0 0.0 0.2 MgO 0.0 0.0 0.2 CaO 1.2 0.9 2.0 SrO 0.0 0.0 0.0 BaO 0.0 0.00.0 ZrO₂ 2.0 1.8 1.9 HfO₂ 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 Ta₂O₅ 0.0 0.00.0 La₂O₃ 0.0 0.0 0.0 Y₂O₃ 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.00.0 ZnO 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0 Total 100.0 100.0100.0 SiO₂ + Al₂O₃ 81.0 82.5 79.5 Li₂O/Na₂O 0.328 0.480 0.182 Li₂O +Na₂O 15.8 14.8 16.2 Li₂O + Na₂O + K₂O 15.8 14.8 16.4 MgO + CaO 1.2 0.92.2 MgO/CaO 0.000 0.000 0.100 SrO + BaO 0.0 0.0 0.0 Liquidus temp. (°C.) 950 950 950 Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV,VI, VII Component Example 12 Example 13 Example 14 SiO₂ 60.8 60.9 65.1Al₂O₃ 17.7 19.5 16.7 Li₂O 2.0 3.9 3.8 Na₂O 14.0 9.2 9.4 K₂O 0.2 0.4 0.3MgO 0.7 0.6 0.6 CaO 1.7 1.5 1.4 SrO 0.0 0.3 0.0 BaO 0.0 0.5 0.0 ZrO₂ 2.92.5 2.7 HfO₂ 0.0 0.0 0.0 Nb₂O₅ 0.0 0.0 0.0 Ta₂O₅ 0.0 0.7 0.0 La₂O₃ 0.00.0 0.0 Y₂O₃ 0.0 0.0 0.0 TiO₂ 0.0 0.0 0.0 Fe₂O₃ 0.0 0.0 0.0 ZnO 0.0 0.00.0 Sb₂O₃ 0.0 0.0 0.0 SO₃ 0.0 0.0 0.0 Total 100.0 100.0 100.0 SiO₂ +Al₂O₃ 78.5 80.4 81.8 Li₂O/Na₂O 0.143 0.424 0.404 Li₂O + Na₂O 16.0 13.113.2 Li₂O + Na₂O + K₂O 16.2 13.5 13.5 MgO + CaO 2.4 2.1 2.0 MgO/CaO0.412 0.400 0.429 SrO + BaO 0.0 0.8 0.0 Liquidus temp. (° C.) 930 940960 Corresponding glass I~IV, VI, VII I~IV, VI, VII I~IV, VI, VIIComponent Example 15 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 SiO₂ 67.4 64.266.0 65.5 Al₂O₃ 11.6 17.2 15.0 17.0 Li₂O 3.9 4.1 3.5 5.0 Na₂O 11.6 9.29.0 8.5 K₂O 0.2 0.4 0.0 0.0 MgO 0.7 0.6 2.5 3.0 CaO 1.8 2.2 2.0 0.0 SrO0.7 1.0 0.0 0.0 BaO 0.0 0.4 0.0 0.0 ZrO₂ 1.8 0.0 1.5 1.0 HfO₂ 0.3 0.00.0 0.0 Nb₂O₅ 0.0 0.0 0.0 0.0 Ta₂O₅ 0.0 0.0 0.0 0.0 La₂O₃ 0.0 0.0 0.00.0 Y₂O₃ 0.0 0.0 0.0 0.0 TiO₂ 0.0 0.12 0.0 0.0 Fe₂O₃ 0.0 0.38 0.0 0.0ZnO 0.0 0.0 0.0 0.0 Sb₂O₃ 0.0 0.0 0.5 0.0 SO₃ 0.0 0.21 0.0 0.0 Total100.0 100.01 100.0 100.0 SiO₂ + Al₂O₃ 79.0 81.4 81.0 82.5 Li₂O/Na₂O0.336 0.446 0.389 0.588 Li₂O + Na₂O 15.5 13.3 12.5 13.5 Li₂O + Na₂O +K₂O 15.7 13.7 12.5 13.5 MgO + CaO 2.5 2.8 4.5 3.0 MgO/CaO 0.389 0.2731.250 — SrO + BaO 0.7 1.4 0.0 0.0 Liquidus temp. (° C.) 940Corresponding glass I~IV, VI, VII — — —

In Table 1, the contents of the various components and their totalcontents are denoted as molar percentages and mass percentages. R₂Odenotes the total content of Li₂O, Na₂O, and K₂O; RO denotes the totalcontent of MgO, CaO, SrO, and BaO; and M denotes the total content ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃ Y₂O₃, and TiO₂. The ratio of the Li₂Ocontent to the Na₂O content, Li₂O/Na₂O, is given as a molar ratio and asa mass ratio. The ratio of the MgO content to the CaO content, MgO/CaO,is also given as a molar ratio and as a mass ratio.

TABLE 2 Example 1 Example 2 Example 3 Example 4 Acid 1.70 1.66 1.75 1.88etching rate [nm/min] Alkali 0.072 0.071 0.070 0.074 etching rate[nm/min]

[Preparation of Glass Substrate for Magnetic Disk and Magnetic Disk -2-]

An example of manufacturing a glass substrate for magnetic disk and amagnetic disk using the glass described in Example 3 above will bedescribed below.

(1) Form Processing Step

Glass of the composition described in Example 3 was molded by directpressing to obtain an amorphous disk-shaped glass substrate.Subsequently, a grindstone was employed to make a hole in the center ofthe glass substrate obtained, yielding a disk-shaped glass substratehaving a round hole in its center. The outer perimeter edge surface andinner perimeter edge surface were then chamfered.

(2) Edge Surface Polishing Step

Next, while rotating the glass substrate, the surface roughness of theedge surfaces (inner perimeter, outer perimeter) of the glass substratewas polished to a maximum height (Rmax) of about 1.0 micrometer and anarithmetic average roughness (Ra) of about 0.3 micrometer with a brushpolisher.

(3) Grinding Step

A grindstone with a grain size of about #1000 was then used to grind thesurface of the glass substrate to a main surface flatness of about 3micrometers, an Rmax of about 2 micrometers, and an Ra of about 0.2micrometer. Here, the term “flatness” means the distance (difference inheight) between the highest portion of the substrate surface and thelowest portion in a perpendicular direction (direction perpendicular tothe surface). Flatness was measures with a flatness measuring device.Rmax and Ra were measured for a square area 5 micrometers on arectangular area with an atomic force microscope (AFM) (a Nanoscope madeby Digital Instruments).

(4) Prepolishing Step

Next, a prepolishing step was conducted with a polishing device capableof polishing the two main surfaces of 100 to 200 glass substrates atonce. A hard polisher was employed as the polishing pad. A polishing padthat had been impregnated in advance with zirconium oxide and ceriumoxide was employed.

The polishing solution employed in the prepolishing step was prepared byadmixing cerium oxide polishing grit 1.1 micrometers in mean graindiameter with water. Polishing grains exceeding 4 micrometers in grainsize were eliminated in advance. When the polishing solution wasmeasured, the maximum value of the polishing grains contained in thepolishing solution was 3.5 micrometers, the average value was 1.1micrometers, and the D50 value was 1.1 micrometers.

Additionally, the load applied to the glass substrate was 80 to 100g/cm², and the reduction in thickness on the surface of the glasssubstrate was 20 to 40 micrometers.

(5) Mirror Polishing Step

The mirror polishing step was conducted with a planetary gear-typepolishing device capable of simultaneously polishing both the mainsurfaces of 100 to 200 glass substrates. A soft polisher was employed asthe polishing pad.

The polishing solution employed in the mirror polishing step wasprepared by adding sulfuric acid and tartaric acid to extremely purewater, followed by the addition of colloidal silica grains having agrain diameter of 40 nm. In this process, the concentration of thesulfuric acid in the polishing solution was adjusted to 0.15 masspercent and the polishing solution was adjusted to pH 2.0 or lower. Theconcentration of tartaric acid was set to 0.8 mass percent and thecontent of colloidal silica grains was set to 10 mass percent.

In the course of the mirror polishing treatment, the pH of the polishingsolution was kept approximately constant, without variation. In thepresent Example, the polishing solution supplied to the surface of theglass substrate was recovered through a drain, cleaned by removal offoreign matter in a meshlike filter, and then reused by supplying itagain to the glass substrate.

The polishing rate during the mirror polishing step was 0.25micrometer/minute. It was found that an advantageous polishing rate wasachieved under these conditions. The polishing rate was determined bydividing the amount of reduction by polishing (processing allowance) inthe thickness of the glass substrate necessary for finishing to aprescribed mirror surface by the time required for polishing.

(6) Cleaning Step Following Mirror Polishing

Next, alkali cleaning was conducted by immersing the glass substrate inan aqueous solution with a 3 to 5 mass percent NaOH concentration.Cleaning was conducted with the application of ultrasound. Cleaning wasthen conducted by sequential immersion in successive cleaning vatscontaining a neutral cleaning agent, pure water, pure water, isopropylalcohol, and isopropyl alcohol (steam drying). Following cleaning, whenthe surface of the glass substrate was observed by AFM (a Nanoscope madeby Digital Instruments) (measurement of a square area 5 micrometers on arectangular area), no adhesion of colloidal silica polishing grit wasdetected. Nor was stainless steel, iron, or any other foreign matterdetected, or any increase in the roughness of the substrate surfaceobserved before and after cleaning.

(7) Chemical Strengthening Treatment Step

Next, the cleaned glass substrate was preheated to 300° C. and immersedfor about 3 hours in an chemical strengthening salt, which had beenobtained by mixing potassium nitrate (60 mass percent) and sodiumnitrate (40 mass percent) and heating the mixture to 375° C., to conducta chemical strengthening treatment. This treatment replaced the lithiumand sodium ions on the surface of the glass substrate with the sodiumions and potassium ions, respectively, in the chemical strengtheningsalt to chemically strengthen the glass substrate. The compressionstress layer formed on the surface of the glass substrate was about 100to 200 micrometers in thickness. After conducting chemicalstrengthening, the glass substrate was quickly cooled by immersion in a20° C. vat of water, where it was kept for about 10 minutes.

(8) Cleaning Step after Chemical Strengthening

Upon completion of the above rapid cooling, the glass substrate wasimmersed in sulfuric acid that had been heated to about 40° C. andwashed while being exposed to ultrasound. Subsequently, the glasssubstrate was washed with a 0.5 percent (Vol %) hydrogenfluosilicic acid(H₂SiF) aqueous solution followed by a 1 mass percent potassiumhydroxide aqueous solution. Glass substrate 12 for magnetic disk wasmanufactured by the above steps.

(9) Step of Inspection of the Glass Substrate for Magnetic Disk

The glass substrate for magnetic disk was then inspected. The surfaceroughness of the glass substrate for magnetic disk was measured by AFM(atomic force microscopy) (a square area 5 micrometers on a rectangulararea was measured), revealing a peak height (Rmax) of 1.5 nm and anarithmetic average roughness (Ra) of 0.15 nm. The surface was in a cleanmirror surface state, without the presence of foreign material impedingflotation of the magnetic head and foreign material causing thermalasperities. No increase in the surface roughness of the substrate wasobserved following cleaning. The deflecting strength was measured next.The deflecting strength was determined using a deflecting strengthmeasuring and testing device (Shimadzu Autograph DDS-2000) as the loadvalue at which the substrate sustained damage when a load was applied tothe glass substrate, as shown in FIG. 2. The deflecting strengthdetected was 24.15 kg, which was a satisfactory value.

In the above description, acid and alkali cleanings were conducted afterchemical strengthening. However, acid and alkali cleanings may beconducted during the cleaning following the mirror polishing step.

Next, magnetic disk 10 was manufactured from substrate 12 comprised ofthe glass of Example 3 and tested in a hard disk drive. FIG. 1 shows aschematic of the film structure (cross-section) on substrate 12.

First, using a film forming device in which a vacuum had been generated,DC magnetron sputtering was used to sequentially form adhesive layer 14and soft magnetic layer 16 in an argon atmosphere.

Adhesive layer 14 was formed using a CrTi target to obtain amorphousCrTi layer 20 nm thick. Soft magnetic layer 16 was formed using a CoTaZrtarget to obtain amorphous CoTaZr (Co: 88 atomic percent, Ta: 7 atomicpercent, Zr: 5 atomic percent) layer 200 nm thick.

Magnetic disk 10 on which the films had been formed up through softmagnetic layer 16 was removed from the film forming device, and thesurface roughness was measured in the same manner as above. A smoothmirror surface with an Rmax of 2.1 nm and an Ra of 0.20 nm was detected.A vibrating sample magnetization (VSM) measuring device was used tomeasure the magnetic characteristics, revealing a coercivity (Hc) of 2Oe and a saturation magnetic flux density of 810 emu/cc. Suitable softmagnetic characteristics were thus exhibited.

A batch static opposition-type film forming device was then employed tosequentially form underlayer 18, granular structure grain size reductionpromoting layer 20, granular structure ferromagnetic layer 32, magneticcoupling control layer 34, exchange energy control layer 36, andprotective film 24 in an argon atmosphere. In the present Example,underlayer 18 had a double-layer structure comprised for a first layerand a second layer.

In this step, a 10 nm layer of amorphous NiTa (Ni: 40 atomic percent,Ta: 10 atomic percent) was initially formed as the first layer ofunderlayer 18 on the disk substrate, followed by a 10 to 15 nm Ru layeras the second layer.

Next, a target comprised of nonmagnetic CoCr-SiO₂ was used to form grainsize reduction promoting layer 20 comprised of a 2 to 20 nm hcp crystalstructure. Then, a target in the form of a hard magnetic materialcomprised of CoCrPt—SiO₂ was used to form ferromagnetic layer 32comprised of a 15 nm hcp crystal structure. The composition of thetarget used to form ferromagnetic layer 32 was: Co: 62 atomic percent,Cr: 10 atomic percent, Pt: 16 atomic percent, and SiO₂: 12 atomicpercent. Magnetic coupling control layer 34 comprised of a Pd layer wasformed, and exchange energy control layer 36 comprised of a [CoB/Pd]nlayer was formed.

Next, CVD in which ethylene was employed as the material gas was used toform protective film 24 comprised of hydrogenated carbon. Sinceproviding hydrogenated carbon increased the hardness of the film,magnetic recording layer 22 was protected from impact by the magneticrecording head.

Subsequently, lubricating layer 26 comprised of PFPE(perfluoropolyether) was formed by dip coating. Lubricating layer 26 was1 nm thick. The above manufacturing steps were used to obtain aperpendicular magnetic recording medium in the form of perpendicularmagnetic recording-mode magnetic disk 10. The surface roughness achievedwas measured in the manner set forth above, revealing a smooth mirrorsurface with an Rmax of 2.2 nm and an Ra of 0.21 nm.

Magnetic disk 10 obtained was placed in a 2.5 inch load-unload type harddisk drive. The magnetic head mounted in the hard disk drive was adynamic flying height (abbreviation: DFH) magnetic head. The floatingheight of the magnetic head relative to the magnetic disk was 8 nm.

A recording and reproduction test that was conducted at a recordingdensity of 200 Gbits/inch² in a recording and reproduction area on themain surface of the magnetic disk using the hard disk drive revealedgood recording and reproduction characteristics. Neither crash failurenor thermal asperity failure occurred during the test.

A load-unload (LUL hereinafter) test was then conducted with the harddisk drive.

The LUL test was conducted with a 2.5-inch hard disk drive rotating at5,400 rpm with the magnetic head floating at 8 nm. The above-describedmagnetic head was employed. The shield element was comprised of NiFealloy. The magnetic disk was loaded into the magnetic disk device andLUL operations were continuously conducted with the above-describedmagnetic head to measure the LUL durability frequency.

Following the LUL durability test, the magnetic disk surface andmagnetic head surface were visually inspected and viewed under anoptical microscope for scratches, grime, and other foreign matter. TheLUL durability test required a durability of no failure during 400,000cycles of LUL, with durability of 600,000 cycles being desirable. In theenvironment in which hard disk drives (HDD) are normally employed,exceeding 600,000 LUL cycles is said to require about 10 years of use.

In the LUL test, magnetic disk 10 achieved a passing score for adurability of 600,000 cycles or more. Inspection of magnetic disk 10following the LUL test revealed no scratches, grime, or other foreignmatter. No precipitation of alkali metal components was found.

Comparative Examples

The three glasses shown in Table 1 were prepared as Comparative Examples1 to 3. Comparative Example 1 was the glass of Example 5 described inJapanese Unexamined Patent Publication (KOKAI) No. 2001-236634.Comparative Example 2 was the glass of Comparative Example 1 describedin Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-232627.And Comparative Example 3 was the glass of Comparative Example 2described in Japanese Unexamined Patent Publication (KOKAI) Heisei No.11-314931.

Since the glass of Comparative Example 1 did not contain ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, or TiO₂, it lacked adequate chemicaldurability, particularly resistance to alkalinity. Since the quantity ofCaO was smaller than the quantity of MgO in the glasses of ComparativeExamples 2 and 3, and since the molar ratio of the total content ofZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃, and TiO₂ relative to the totalcontent of Li₂O, Na₂O, K₂O, MgO, CaO, SrO, and BaO was low, chemicaldurability was inadequate.

The present invention can provide a magnetic recording medium that issuited to high-density recording such as that for perpendicular magneticrecording systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing an example of the structure of the magneticdisk according to an implementation embodiment of the present invention.

FIG. 2 is a descriptive drawing of a method for measuring deflectingstrength.

1. Glass for use in substrate for information recording medium, whichcomprises, denoted as molar percentages, 50 to 75 percent of SiO₂; 3 to15 percent of Al₂O₃; 5 to 15 percent of Li₂O; 5 to 15 percent of Na₂O; 0to 3 percent of K₂O; greater than 0.5 percent but equal to or less than5 percent of CaO; equal to or greater than 0 percent but less than 3percent of MgO, with CaO content being greater than MgO content; and 0.3to 4 percent of ZrO₂; with the molar ratio of the total content of Li₂O,Na₂O and K₂O to the total content of SiO₂, Al₂O₃ and ZrO₂((Li₂O+Na₂O+K₂O)/(SiO₂+Al₂O₃+ZrO₂)) being equal to or less than 0.28. 2.Glass for use in substrate for information recording medium, whichcomprises SiO₂; Al₂O₃; one or more alkali metal oxides selected from thegroup consisting of Li₂O, Na₂O and K₂O; one or more alkaline earth metaloxides selected from the group consisting of MgO, CaO, SrO and BaO; andone or more oxides selected from the group consisting of ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂; wherein SiO₂ content is equal to orgreater than 50 molar percent, and the total content of SiO₂ and Al₂O₃is equal to or greater than 70 molar percent; the total content of saidalkali metal oxides and said alkaline earth meal oxides is equal to orgreater than 8 molar percent; and the molar ratio of the total contentof said oxides to the total content of said alkali metal oxides and saidalkaline earth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO)) isequal to or greater than 0.035.
 3. The glass according to claim 2,wherein SiO₂ content is equal to or greater than 60 molar percent, andthe total content of SiO₂ and Al₂O₃ is equal to or greater than 75 molarpercent.
 4. The glass according to claim 3, which comprises at least oneof Li₂O and Na₂O, and wherein the total content of Li₂O and Na₂O is lessthan 24 molar percent.
 5. The glass according to claim 2, wherein thetotal content of Li₂O and Na₂O is equal to or less than 22 molarpercent.
 6. The glass according to claim 2, which comprises, denoted asmolar percentages, 60 to 75 percent of SiO₂; 3 to 15 percent of Al₂O₃;and 0.3 to 4 percent of ZrO₂.
 7. Aluminosilicate glass for chemicalstrengthening for use in substrate for information recording medium,which comprises: one or more alkali metal oxides selected from the groupconsisting of Li₂O, Na₂O and K₂O, one or more alkaline earth metaloxides selected from the group consisting of MgO, CaO, SrO and BaO, andone or more oxides selected from the group consisting of ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂; wherein the total content of Li₂Oand Na₂O is 10 to 22 molar percent; the total content of ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ is greater than 0 molar percent butequal to or less than 4 molar percent; and the molar ratio of the totalcontent of said oxides to the total content of said alkaline earth metaloxides ((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(MgO+CaO+SrO+BaO)) isequal to or greater than 0.15.
 8. The glass according to claim 7,wherein SiO₂ content is equal to or greater than 50 molar percent, andthe total content of SiO₂ and Al₂O₃ is equal to or greater than 70 molarpercent.
 9. The glass according to claim 6, wherein the total content ofSiO₂ and Al₂O₃ is equal to or greater than 75 molar percent.
 10. Theglass according to any of claim 7, wherein the molar ratio of the totalcontent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ to the totalcontent of Li₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035.
 11. The glass according to claim 7,which comprises, denoted as molar percentages, equal to or greater than3 percent of Al₂O₃; a total of equal to or greater than 8 percent ofLi₂O, Na₂O, K₂O, MgO, CaO, SrO and BaO; and a total of greater than 0percent but equal to or less than 5 percent of MgO, CaO, SrO and BaO.12. Glass for use in substrate for information recording medium, whichcomprises SiO₂; Al₂O₃; one or more alkali metal oxides selected from thegroup consisting of Li₂O, Na₂O and K₂O; one or more alkaline earth metaloxides selected from the group consisting of MgO, CaO, SrO and BaO; andone or more oxides selected from the group consisting of ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂; which has an acidity resistanceresulting in an etching rate of equal to or less than 3.0 nm/minute whenimmersed in 0.5 volume percent hydrogenfluosilicic acid (H₂SiF) aqueoussolution maintained at 50° C.; and which has an alkalinity resistanceresulting in an etching rate of equal to or less than 0.1 nm/minute whenimmersed in 1 mass percent potassium hydroxide aqueous solutionmaintained at 50° C.
 13. The glass according to claim 12, wherein SiO₂content is equal to or greater than 50 molar percent, and the totalcontent of SiO₂ and Al₂O₃ is equal to or greater than 70 molar percent.14. The glass according to claim 12, wherein the total content of SiO₂and Al₂O₃ is equal to or greater than 75 molar percent.
 15. The glassaccording to claim 12, which has a composition that the total content ofsaid alkali metal oxides and said alkaline earth metal oxides is equalto or greater than 8 molar percent, and the molar ratio of the totalcontent of said oxides to the total content of said alkali metal oxidesand said alkaline earth metal oxides((ZrO₂+HfO₂+Nb₂O₅+Ta₂O₅+La₂O₃+Y₂O₃+TiO₂)/(Li₂O+Na₂O+K₂O+MgO+CaO+SrO+BaO))is equal to or greater than 0.035.
 16. The glass according to any ofclaim 12, which comprises at least one of Li₂O and Na₂O, and wherein thetotal content of Li₂O and Na₂O is equal to or less than 24 molarpercent.
 17. The glass according to any of claim 12, wherein the totalcontent of Li₂O and Na₂O is equal to or less than 22 molar percent. 18.The glass according to any of claim 12, which comprises, denoted asmolar percentages, 60 to 75 percent of SiO₂; 3 to 15 percent of Al₂O₃;and a total of 0.3 to 4 percent of ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃and TiO₂.